You see a warning like the following within a mailbox alert in Dynamics 365:
"A general mailbox access error occurred while sending the email message Test SSS with incoming via EXO profile but really on-prem through the mailbox <Mailbox Name>. The owner of the associated email server profile Microsoft Exchange Online has been notified. The system will try to send the message again later.
Email Server Error Code: MailboxNotEnabledForRESTAPI. Exchange.server returned 404 error."