You might encounter an issue where your Kayako Classic instance has stopped receiving emails or messages from your Microsoft Office 365 email address. The following error may be observed:
Delivery has failed to these recipients or groups:
Your message wasn't delivered because the recipient's email provider rejected it.
The error might be a little misleading but the real cause of this is because Microsoft has recently rolled out the External Email Forwarding Controls functionality for customers with Office 365 Advanced Threat Protection (ATP) licensed. The update will affect forwarding to external domains because the new default setting is disabled. Hence, if you are using an Office 365 email address as your email channel in Kayako, you might notice that suddenly no emails are coming and no conversations are being created as well.
The fix is to re-enable automatic forwarding. Check out the Configuring external email forwarding in Office 365 article from Microsoft for more information about this topic. Should you need further assistance, please contact the Microsoft Support team.