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

This article provides a solution to an error that occurs when you try to test and enable a mailbox in Microsoft Dynamics 365.

Applies to:   Microsoft Dynamics 365
Original KB number:   4092733


When you attempt to test and enable a mailbox in Microsoft Dynamics 365, you receive 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 select the Details section, you may also see more 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 isn't correct.


To fix this issue, follow these steps:

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

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


    You can also select the name of the Email Server Profile within the error message. It appears as a hyperlink which will open the correct Email Server Profile record.

  3. Select Advanced.

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


    Refer to the documentation from your email provider for the correct ports to use when connecting to the SMTP service. For Gmail the port is 587.

  5. Select Mailboxes and then select the mailbox that received the error.

  6. Select the Test & Enable Mailboxes button.

  7. After the tests complete, open the mailbox record and view the Alerts section if the results don't appear as Success.