If you have received a Delivery failed because the sender isn't a group member error message when trying to send an email from Front, it may have been caused by the configuration of the Office 365 mailbox you are trying to send that message to. 

This error is received because the distribution group you are emailing might not allow certain individuals to send email to the group.

To resolve this, we would recommend contacting the recipient's IT team to ensure that the intended individuals can send messages to that distribution group. Details on how to do that are included in this help article from Microsoft: Office 365 Group not allowing external senders - Accept Messages from all Senders is selected