If you were viewing an alert within a Dynamics 365 mailbox and clicked the Learn More link next to a message referencing an Exchange error, you may be redirected to this article. For example: You may see a message that contains the following text followed by an error number. Clicking the Learn More link at the end of the message may direct you to this article.
"Email Server Error Code: Exchange server returned..."
If your Dynamics 365 organization is configured with Microsoft Exchange, Dynamics 365 will communicate with Exchange using a web service such as Exchange Web Services (EWS). If Dynamics 365 sends a request to Exchange and Exchange returns a failure message, that error may be logged in the Alerts section of the corresponding Mailbox record in Dynamics 365. If you were directed to this article, it indicates the Dynamics 365 team does not yet have an article focused on that specific Exchange error.
Although Dynamics 365 does not yet have an article for this error code returned by Exchange, you can refer to the following Microsoft Exchange article for a description of the different error codes returned by Exchange Web Services (EWS):
For example: If the error code was 363, the article above describes this as the ErrorServerBusy message and documents this "Occurs when the server is busy". So this may indicate that Dynamics 365 tried to retrieve information from Exchange but Exchange indicated it was currently to busy to process the request. Something like this can indicate Exchange is busy processing other requests and retrying the request later may succeed.