When viewing the Alerts section of a mailbox record in Dynamics 365, you see the following warning alert:
"Appointments, contacts, and tasks for the mailbox [Mailbox Name] couldn't be synchronized. The owner of the associated email server profile [Profile Name] has been notified. The system will try again later.
Email Server Error Code: Exchange.server returned 203 error."
This message can normally be ignored and does not impact synchronization. Microsoft is working on a fix to make this warning less likely to appear. There are still some scenarios were this may appear. One example is when a non-organizer is trying to update required attendees on an appointment.
If this warning appears but you are not seeing any issues with synchronization, this warning can normally be ignored. As mentioned in the Cause section, Microsoft is working on a fix to reduce how often this appears and it should not impact synchronization.