How to navigate SMTP error codes

Encountering an email delivery error can be frustrating, but these codes provide valuable clues about what went wrong. This guide will help explain common SMTP (Simple Mail Transfer Protocol) error codes and how to address them.

Email errors fall into two main categories: permanent failures (hard bounces) and temporary failures (soft bounces).

• 5xx codes (permanent failures / hard bounces): These codes signify a permanent error, and the server will not attempt to resend the message.

• 4xx codes (temporary failures / soft bounces): These codes indicate a temporary problem, and usually means the sender can try sending the message to the recipient again.

While there are some common definitions for SMTP error codes, interpretations vary significantly across mail providers, and codes can have multiple, nuanced meanings.

• Verify the email address is valid: The recipient may no longer be able to receive emails due to the permanent failure. Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

• Report consistent issues: If you notice a pattern of hard bounces for a specific domain or a large number of emails, report this to your system administrator. If you are able to get in touch with the system administrator of the recipient, they may be able to assist you as well.

• Adjust content: If you receive notifications that emails are rejected due to spam content (e.g., for codes like 554 or 561), try modifying your email content to avoid common spam triggers (e.g., excessive links, suspicious attachments, all caps, common spam phrases).

• Retry sending to the recipient later: Create a copy of the draft message to send to that specific recipient. These errors should go away over time allowing you to eventually send successfully.

• Monitor consistent soft bounces: Keep an eye on email addresses that consistently soft bounce (e.g., are always full). If an address repeatedly bounces, it might eventually be treated as a hard bounce.

• Report widespread issues: If you observe an unusually high volume of temporary failures, especially if they are affecting multiple recipients or domains, notify your system administrator. This could indicate a broader issue with your recipient's domain that requires administrative intervention.

• Adjust sending practices (if applicable): If your role allows you to control sending volume or frequency, and you're seeing "too busy" or "rate-limited" errors (e.g., 423, 435), consider if you can space out your sends to avoid overwhelming recipient servers. 

These errors mean your email could not be delivered at all, and our system will not try to send it again. The issue is often a persistent problem with the recipient's email address or their server setup that requires them to take action to resolve.

These error codes suggest an unspecified or generic error occurred on the recipient's end. The receiving server did not understand a command it received or could not provide a more specific reason for the failure.

• Common causes: Server misconfiguration on the recipient's side, a network error, or an unrecognized command.

• Recommended actions: The recipient may no longer be able to receive emails due to the permanent failure. Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

These codes indicate that the recipient's email address doesn't exist at the domain, often because the user has left the company. It can also point to a typo in the recipient's email address.

• Common causes: The user's account has been deleted, or there's a typo in the email address.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

This code represents a general, non-temporary failure, often indicating an unexpected issue that prevents delivery.

• Common causes: This is a generic code that can be used for various permanent failures, sometimes related to the sender's authentication or server policies on the receiving end.

• Recommended actions: Work with your system administrator to check for potential sending issues. The recipient’s administrator may also need to investigate.

These error codes suggest the recipient's email address is incorrect or does not exist at their provider. It is similar to sending a physical letter to an address that doesn't exist.

• Common causes: A typo in the email address or the contact's email account has been deleted.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

The domain part of the email address (e.g., "company.com") does not exist, is misspelled, or is not configured to receive mail.

• Common causes: A typo in the domain name, the domain has expired, or a DNS (Domain Name System) misconfiguration on the recipient's end.

• Recommended actions: Verify the domain name is spelled correctly. If it is, the recipient's domain administrator needs to fix their server records.

These error codes suggest there is a syntax error in the email address, such as a missing "@" symbol, extra spaces, or invalid characters.

• Common causes: The email address was typed incorrectly or copied and pasted with hidden characters.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

The recipient's mailbox has been moved, and no forwarding address is available.

• Common causes: The user has changed their email address, often due to a change in roles or leaving the company, and an automatic forward was not set up.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

The sender's email address has a syntax error or is invalid.

• Common causes: The sender's email address is not properly formatted or contains invalid characters.

• Recommended actions: Validate the "From" address used when sending the email is valid.

• The sender's domain is invalid.

• Common causes: A misconfiguration in your organization's DNS records.

• Recommended actions: This issue must be resolved by your system administrator by correcting the domain's DNS settings.

The recipient's mailbox is unable to receive messages, possibly because it is inactive or disabled.

• Common causes: The recipient's email account has been suspended or is not currently active.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

The recipient's mailbox is disabled or isn't accepting new messages.

• Common causes: The account may be suspended or undergoing maintenance.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

These error codes suggest the recipient's mailbox is full and cannot accept more messages.

• Common causes: The recipient has exceeded their storage quota.

• Recommended actions: Contact the recipient via another method to inform them that their inbox is full so they can clear space.

These error codes suggest your email is too large for the recipient's server to accept because it exceeds the maximum size allowed.

• Common causes: The email contains very large attachments or embedded images.

• Recommended actions: Try modifying your email content to have less attachments and images.

These error codes suggest the recipient's system is not accepting messages, or the entire domain does not accept mail.

• Common causes: The organization may no longer be accepting email at this domain, or there is a major server outage or policy change.

• Recommended actions: Work with your system administrator to verify that the organization still uses this domain for email communication.

The recipient's server refused the email, often because your email address or sending IP is on a blocklist or due to a policy violation.

• Common causes: Your emails have been marked as spam, or the recipient's server has a strict policy against your domain or IP.

• Recommended actions: Contact your system administrator to investigate if your domain/IP is on a blocklist. You may also need to contact the recipient's administrator to request removal.

These error codes suggest the message could not route to the destination mailbox.

• Common causes: The sending server has misconfigured or non-existent MX (Mail eXchanger) records in its DNS settings, or the sending server has an incorrect configuration.

• Recommended actions: Work with your system administrator to confirm that the sending settings are correct. Have the administrator validate the MX configuration and DNS settings.

These error codes suggest that network congestion is preventing delivery, and the server is treating it as a permanent failure. This can also indicate that a daily sending limit has been exceeded.

• Common causes: Severe and persistent network problems between the sending and receiving servers. 

• Recommended actions: This is typically outside of your control. You can try sending the email again later.

These error codes suggest the email encountered an endless forwarding loop between servers.

• Common causes: A misconfiguration of email forwarding rules on either the sender's or recipient's server.

• Recommended actions: This is a server configuration problem. Inform your administrator and, if possible, the recipient's administrator.

These codes indicate persistent DNS or network routing problems that prevent the server from finding the recipient's domain.

• Common causes: The recipient's domain has incorrect DNS records, or there are significant network issues.

• Recommended actions: This is a technical issue on the recipient's side that must be fixed by their domain administrator.

This code indicates that the two mail servers involved in the transfer were unable to successfully communicate using the SMTP protocol, and the problem is not covered by any other more specific error code.

• Common causes: There is an issue with the recipient's server such as being on older software, or it has a configuration issue.

• Recommended actions: Work with your administrator to see if there are alternate ways to reach the recipient.

The server did not understand the request because of a syntax error or unsupported parameters.

• Common causes: An issue with the sending email client or server that is sending non-standard commands.

• Recommended actions: Work with your administrator to validate the sending settings such as SPIF and DKIM are correct.

The number of recipients exceeds the server's limit.

• Common causes: You have added too many email addresses in the To, Cc, or Bcc fields.

• Recommended actions: Reduce the number of recipients in the email and resend it in smaller batches.

This represents a general transaction failure or message rejection.

• Common causes: A generic code that can be tied to various policy or technical issues.

• Recommended actions: Work with your administrator to validate the sending settings such as SPIF and DKIM are correct. If they are correct, work with your administrator to see if there are alternate ways to reach the recipient.

These error codes suggest the message content is not supported or acceptable to the recipient's system.

• Common causes: The email may contain a format, encoding, or content type that the receiving server does not permit.

• Recommended actions: Try sending the email in a simpler format (e.g., plain text) or removing attachments to see if it can be delivered.

These error codes suggest that ender authentication failed due to invalid credentials.

• Common causes: The sending system requires authentication (username/password), and the provided credentials were incorrect.

• Recommended actions: This is a technical issue. Your system administrator should check the configuration of the sending channel.

These error codes suggest the email was rejected because it failed a DMARC policy check.

• Common causes: DMARC is a security measure to prevent email spoofing. This failure means your email did not align with your domain's stated policies for handling unauthenticated mail.

• Recommended actions: Your system administrator must ensure that your SPF and DKIM records are correctly configured and aligned with your DMARC policy.

These error codes suggest the message could not be delivered within the allowed time and is now considered a permanent failure.

• Common causes: Persistent temporary failures (like the recipient's server being down) eventually timed out and were converted to a hard bounce.

• Recommended actions: Treat this as a persistent issue. Try contacting the recipient through another channel.

The receiving mail server has explicitly refused to accept the message for this specific recipient.

• Common causes: The two main reasons for failure are having too many recipients on the email message or the recipient domain is not configured properly.

• Recommended actions: Check the message with the code. If it mentions too many recipients, try sending the message with just this recipient. If it mentions the failure on the recipient side, work with your system administrator to let the recipient know they may not receive messages.

This error suggests that the mail service has been intentionally stopped or has entered a state where it will not even begin to accept connections for message delivery.

• Common causes: The receiving email server is going through system maintenance or has stopped running.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, try resending your email to the recipient. If it still fails, work with your administrator to see if there are alternate ways to reach the recipient and let them know that they might not be able to receive any messages.

The sender's mail server successfully connected to the recipient's server, but the connection was then rejected or terminated prematurely by the receiving system

• Common causes: The sending server IP address is on a block list, the message got filtered by anti-spam measures, or the receiving server refused the connection.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

The receiving mailbox is unavailable.

• Common causes: The recipient's mailbox doesn't exist, or their server simply won't accept mail from you for policy reasons, potentially due to spam filtering.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

There's something wrong with the recipient's email address format or username that the server doesn't allow.

• Common causes: the recipient's email address contains invalid characters, is missing the @ symbol, or does not conform to the syntax the server requires.

• Recommended actions: Check that the email address in your contact list is valid. If the email address is valid and the recipient is still with the company, work with your administrator to see if there are alternate ways to reach the recipient.

The receiving server does not support the parameters to accept the sent message.

• Common causes: The sending mail server is trying to use a command or a parameter with a command that the receiving server does not understand or support. 

• Recommended actions: Work with your administrator to see if there are alternate ways to reach the recipient.

The recipient's domain does not accept mail.

• Common causes: A valid route to the domain was found, but the server at that destination is explicitly configured to not accept any mail for that domain. This can also happen during a mail provider migration if an old server is still receiving mail but is set to reject it.

• Recommended actions: Work with your administrator to see if there are alternate ways to reach the recipient.

The content of the message is not acceptable.

• Common causes: The content of the message violates a specific policy. This is often used by gateways that inspect message content for sensitive information (Data Loss Prevention), policy violations (e.g., inappropriate language), or specific disallowed file types within a ZIP archive.

• Recommended actions: Remove any attachments, confidential data, or content that might be triggering the filter. Try sending a very simple, plain-text message to see if it goes through.

Sender authentication failed due to invalid credentials.

• Common causes: The credentials provided to send the email were incorrect.

• Recommended actions: Check the password is still valid and update it if it is incorrect.

This is a general message rejection. The server is rejecting the message for a reason that doesn't fit any other code. It's a generic "no" and requires reading the full bounce message for context.

• Common causes: This code is a generic code that captures a wide range of errors.

• Recommended actions: Work with your system administrator to look at the error message and troubleshoot the cause.

These errors indicate a temporary issue. Front will attempt to resend the email once. If you are seeing this error when sending, try sending to the recipient at a later time.

A general temporary problem prevented immediate delivery.

• Common causes: This is a non-specific code used when the recipient's server is temporarily unavailable for an unknown reason.

• Recommended actions: Retry sending to the recipient later: Create a copy of the draft message to send to that specific recipient.

These error codes suggest that authentication is needed to proceed with sending the message.

• Common causes: The server requires authentication, but none was provided.

• Recommended actions: Your system administrator should check the configuration of the sending channel to ensure it authenticates correctly.

The email delivery timed out after several retry attempts.

• Common causes: A persistent temporary issue is preventing delivery.

• Recommended actions: This is still considered a temporary failure, but if it continues, it may become a permanent bounce. Monitor the address for future failures.

These error codes suggest the recipient's server or mailbox is temporarily unavailable or not accepting messages.

• Common causes: The server could be down for maintenance, overloaded, or the mailbox may be temporarily disabled.

• Recommended actions: It is best to treat this as a hard bounce and verify the address.

These error codes suggest the connection has had too many errors and is being dropped.

• Common causes: The sending server has made too many failed attempts or errors in a single session.

• Recommended actions: Retry sending to the recipient later. If the email message was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest an unspecified temporary error occurred.

• Common causes: The receiving server encountered a temporary condition it could not classify.

• Recommended actions: Retry sending to the recipient later. If the email message was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest the sender is sending too many messages or sending too fast and is being temporarily rate-limited (throttled).

• Common causes: Sending a large volume of emails in a short period to the same server.

• Recommended actions: Retry sending to the recipient later. If the email message was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest the recipient's mailbox is temporarily unavailable.

• Common causes: The mailbox might be undergoing maintenance or is temporarily over its storage limit.

• Recommended actions: Retry sending to the recipient later. If the email message was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest the recipient's mail server is temporarily unavailable or too busy.

• Common causes: The server is overloaded with connections or is undergoing maintenance.

• Recommended actions: Retry sending to the recipient later. If the email was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest the recipient's inbox is full or the mail system has temporarily run out of storage space.

• Common causes: The user has exceeded their storage quota.

• Recommended actions: Retry sending the message to the recipient. If the issue persists, the recipient needs to clear space in their mailbox.

These error codes suggest there is a temporary issue establishing a secure (TLS) connection.

• Common causes: A temporary problem with the security certificates or protocols on either server.

• Recommended actions: Retry sending to the recipient later. If the email was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest the sender has sent too many messages or to too many recipients in the current session.

• Common causes: Exceeding the server's limits for a single connection.

• Recommended actions: If possible, reduce the number of recipients per batch.

These error codes suggest the recipient's overall email system is temporarily full or overloaded.

• Common causes: The entire mail server is too busy or has no available storage.

• Recommended actions: Retry sending to the recipient later. If the email message was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

The connection to the recipient's email server timed out.

• Common causes: A temporary network issue between the servers or the recipient's server is down or not responding.

• Recommended actions: Retry sending to the recipient later. If the email was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest there is a temporary issue with the message header.

• Common causes: The header is malformed or does not meet the recipient server's temporary policies.

• Recommended actions: This is a technical issue. Report it to your system administrator.

These error codes suggest a temporary network connection or routing problem occurred.

• Common causes: A temporary DNS issue or a routing loop was detected.

• Recommended actions: Retry sending to the recipient later. If the email was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

The email delivery timed out after several retry attempts.

• Common causes: A persistent temporary issue is preventing delivery.

• Recommended actions: This is still considered a temporary failure, but if it continues, it may become a permanent bounce. Monitor the address for future failures.

The recipient's server experienced a temporary internal problem or policy issue.

• Common causes: A local issue on the receiving server prevented it from processing the email.

• Recommended actions: Retry sending to the recipient later. If the email was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest there is a temporary authentication issue when sending the email.

• Common causes: The sender's authentication credentials could not be temporarily verified.

• Recommended actions: Your system administrator may need to investigate the sending channel's authentication settings.

These error codes suggest the message is temporarily considered too large.

• Common causes: The recipient's server has a temporary policy restricting message size.

• Recommended actions: Try modifying the content or attachments and resending.

These error codes suggest the recipient's mailbox is not accessible at the moment, or there is a temporary policy issue.

• Common causes: A temporary problem on the recipient's side, often related to security policies.

• Recommended actions: Retry sending to the recipient later. If the email was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest the server is temporarily not relaying messages.

• Common causes: The recipient's server is configured to not accept third-party emails at this time.

• Recommended actions: Retry sending to the recipient later. If the email was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest there is a temporary issue with DNS resolution.

• Common causes: A temporary failure to look up the recipient's domain information.

• Recommended actions: Retry sending to the recipient later. If the email was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient.

These error codes suggest this indicates a temporary policy rejection.

• Common causes: Your message temporarily violates a policy on the receiving server.

• Recommended actions: Retry sending the email to the recipient.  If this error persists, it may be treated as a permanent failure.

These error codes suggest the message is temporarily deferred by the recipient server.

• Common causes: A generic temporary rejection, often used by anti-spam systems.

• Recommended actions: Retry sending to the recipient later. If the email was for the one recipient, retry sending through Front. If there are multiple recipients, create a copy of the draft message to send to that specific recipient. 

These error codes suggest there is a temporary issue related to DMARC.

• Common causes: A temporary failure during the DMARC authentication check.

• Recommended actions: Your system administrator should ensure DMARC is correctly configured.

Many 5.7.x codes fall into this category, often indicating a hard bounce due to spam filtering or policy. Some 4.7.x codes can indicate temporary blocks or greylisting.

These error codes suggest the recipient's server is temporarily deferring your email, often due to "greylisting".

• Common causes: Greylisting is an anti-spam technique where an unknown sender is asked to try again later. Legitimate servers will, while most spam tools will not.

• Recommended actions: Retry sending to the recipient later: Create a copy of the draft message to send to that specific recipient. 

A generic temporary system error on the receiving end related to security policies.

• Common causes: A catch-all code for a temporary anti-spam or policy issue.

• Recommended actions: Retry sending to the recipient later: Create a copy of the draft message to send to that specific recipient. 

These error codes suggest the sender might be temporarily blocked due to suspicious activity.

• Common causes: Sending patterns that appear spam-like have triggered a temporary block.

• Recommended actions: Wait for the block to be lifted. If you are sending mass emails, review your sending practices.

A generic refusal, often due to a security policy or the message being flagged as spam.

• Common causes: This is a broad code for messages blocked for policy reasons.

• Recommended actions: Review the content for spam triggers. Your system administrator may need to contact the recipient's administrator to understand the specific policy.

These error codes suggest an error indicating the email was rejected for security reasons, such as being flagged as spam, the sender's IP or domain being on a blocklist, or the sender not being authorized.

• Common causes: Poor sender reputation, failing authentication checks (SPF/DKIM), or content flagged as spam.

• Recommended actions: Check if your domain or IP is on a public blocklist. Ensure your email content is not spammy. Your system administrator should verify that SPF and DKIM records are correctly set up.

These error codes suggest the recipient's email address is not known or not recognized by their system.

• Common causes: The email address is incorrect or has been disabled. It also suggests that the email is being bounced between servers endlessly due to a misconfiguration of email forwarding rules. Because the email can never be delivered, it results in a hard bounce.

• Recommended actions: If the email address is incorrect, verify that it is spelled correctly. If it is spelled correctly, notify your system administrator, who may need to coordinate with the recipient's administrator to find and fix the loop.

The recipient's email account has been disabled.

• Common causes: The user's account is no longer active.

• Recommended actions: Treat this as a permanent failure. Remove the address from your contact list.

The email failed the SPF (Sender Policy Framework) check.

• Common causes: Your domain's SPF record is missing or does not include the IP address of the server sending the email. This suggests the email might be forged242.

• Recommended actions: This is a technical issue. Your system or domain administrator must create or correct the SPF record in your domain's DNS settings.

The email failed the DKIM (DomainKeys Identified Mail) check.

• Common causes: The email's digital signature did not validate, suggesting the message may have been altered or is not from your domain. It can also be caused by a misconfiguration.

• Recommended actions: Your system administrator must ensure that DKIM is correctly configured for your sending domain.

These error codes suggest the message was rejected because it violated a specific security policy set by the recipient's provider.

• Common causes: The email triggered a specific rule related to spam, attachments, or sender reputation.

• Recommended actions: The cause is a strict policy on the recipient's side. Try to simplify the message content or contact the recipient another way.

These error codes suggest these errors are often related to anti-spam measures.

• Common causes: The sending IP has a poor reputation or the message has characteristics of spam.

• Recommended actions: Your system administrator should investigate your sending server's reputation and check for blocklistings.

These error codes suggest these indicate the email was caught by strict spam filters, often due to sender reputation issues or advanced security policies.

• Common causes: Your domain or IP address may have a history of sending spam, triggering advanced filters.

• Recommended actions: These are serious reputation issues. Your administrator needs to identify the cause of the poor reputation and may need to contact the recipient's provider to resolve it.

The sender's email address has been blocked or suspended by the receiving server. This is often due to policy violations, security concerns, or reputational issues.

• Common causes: Your email address may be in a blacklist, violating security policies of the recipient domain, or the email is marked as suspicious.

• Recommended actions: Work with your system administrator to address your email address's reputation.

Your message was not delivered because the recipient's system believes it is spam.

• Common causes: The recipient's mail server has your email on a blacklist, marked your email with poor sender reputation, or treated the message as spam-like content.

• Recommended actions: Work with your system administrator to contact the Recipient directly as the block is managed on their end.

This is a generic but very common rejection code, often used when a message is definitively flagged as spam.

• Common causes: The recipient mail server has high confidence that your message is spam.

• Recommended actions: Review the message content, and work with your system administrator to check for black lists and verify settings such as SPIF, DKIM, DMARC.

• Blocklist (or Blacklist): A real-time list of domains or IP addresses that are known for sending spam. Email servers use these lists to reject messages from untrustworthy sources.

• DKIM (DomainKeys Identified Mail): Think of this as a digital, tamper-proof seal on an email. It adds a unique signature to your messages that allows the recipient's server to verify that the email actually came from you and wasn't altered in transit.

• DMARC (Domain-based Message Authentication, Reporting, and Conformance): An email security policy that builds on SPF and DKIM. It tells receiving email servers what to do if a message claims to be from your domain but fails the SPF or DKIM checks (for example, reject the message or send it to spam). It helps prevent email spoofing and phishing.

• DNS (Domain Name System): The internet's address book. It translates human-friendly domain names (like www.google.com) into computer-friendly IP addresses (like 142.250.217.142). For email, DNS is crucial for directing messages to the correct server.

• Forwarding loop: An endless email cycle where a message is passed back and forth between two or more mailboxes. This usually happens when Mailbox A is set to forward to Mailbox B, which is then set to forward back to Mailbox A.

• Greylisting: A spam-fighting technique where a receiving mail server temporarily rejects an email from an unknown sender. Legitimate mail servers will always try to send the email again after a short delay, while many spamming tools will not. When the sending server tries again, the greylisting server will accept the email.

• Mail server: A specialized computer system that acts like a virtual post office. It sends, receives, and stores emails for a particular domain.

• MX (Mail eXchanger) record: A specific type of DNS record that tells the internet where to deliver emails for a particular domain. If the MX record is incorrect or missing, emails can't be delivered.

• Protocol: A set of rules that computers use to communicate with each other. SMTP is the protocol used for sending emails.

• SMTP (Simple Mail Transfer Protocol): This is the standard language, or protocol, that mail servers use to send emails to each other across the internet. Think of it as the digital postman responsible for carrying your message from your server to the recipient's server.

• SPF (Sender Policy Framework): A list published by a domain owner that specifies which mail servers are authorized to send emails on behalf of that domain. It's like a public list of approved company messengers, which helps receiving servers verify that an email is not a forgery.

• Syntax error: An error caused by a typo or incorrect formatting in an email address or a command given to a mail server. For example, john.doe@.com or john.doe@com would be syntax errors because they are missing parts of a valid address.