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.