Hi Jeff! Isaiah here from the Support team. 

You're receiving this error message because the mailbox storage quota for this address has been exceeded within Office 365. You won't be able to send or receive messages until more storage space is cleared up. We do not implement storage quotas at Front, but because your Front account lays on top of Office 365 as a mail provider, this is impacting your ability to send and receive emails in Front as well.

To resolve this, I would suggest deleting/archiving emails within the Office 365 mailbox, or asking your IT admins to increase the storage size for this account.

For more information, please click here for Outlook's support page centered around Storage Limits on Outlook: https://support.microsoft.com/en-us/office/storage-limits-in-outlook-com-7ac99134-69e5-4619-ac0b-2d313bba5e9e?ui=en-us&rs=en-us&ad=us

Please let me know if I can answer any other questions!

Thank you.  We found the account that was the culprit and increased our storage.