"An unknown error occurred while sending the email message" error appears when testing and enabling a mailbox in Dynamics 365

When you attempt to test and enable a mailbox in Microsoft Dynamics 365, you encounter the following error:

"An unknown error occurred while sending the email message "Your mailbox is now connected to Dynamics 365". Mailbox <Mailbox Name> didn't synchronize. The owner of the associated email server profile <Profile Name> has been notified."

If you click the Details section, you may also see addition details such as the following text:

"Error : System.Net.Mail.SmtpException: Syntax error, command unrecognized. The server response was: 

   at System.Net.Mail.SmtpConnection.ConnectAndHandshakeAsyncResult.End(IAsyncResult result)
   at System.Net.Mail.SmtpTransport.EndGetConnection(IAsyncResult result)
   at System.Net.Mail.SmtpClient.ConnectCallback(IAsyncResult result)"

This error can occur if the Outgoing Port value within the Advanced section of the email server profile is not correct.

1. Navigate to Settings, Email Configuration, and then click Email Server Profiles.

2. Open the Email Server Profile that is used by the mailbox encountering this error.

3. Click Advanced.

4. Correct the Outgoing Port value and then click Save.

5. Click Mailboxes and then select the mailbox that encountered the error.

6. Click the Test & Enable Mailboxes button.

7. After the tests complete, open the mailbox record and view the Alerts section if the results do not appear as Success.