Emails are not received in a Dynamics 365 mailbox and you see a warning alert like the following in a user's mailbox record:
"An unknown error occurred while receiving email through the mailbox [Mailbox Name]. The owner of the associated email server profile [Profile Name] has been notified. The system will try to receive email again later.
Email Server Error Code: Exchange server returned UnknownIncomingEmailIntegrationError -2147167669 exception."