An email sent to a Dynamics 365 mailbox is not created as an email activity record and the following message appears as an alert in the user's Dynamics 365 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 -2147218677 exception."
This error occurs if there was an invalid email address included on the To or Cc line of the email.
Review the email message that encountered this error and verify all email addresses on the To and Cc lines are valid email addresses.