XCON: Messages Back Up in the Queue When You Forward Mail with Unresolved Recipients to the Host

When you configure a Simple Mail Transfer Protocol (SMTP) virtual server to forward all mail that contains unresolved recipients to the host, and then specify a third-party SMTP server or a Exchange Server 5.5 computer, the messages may back up in the host queue, and may not be delivered.

This issue can occur if the third-party SMTP server or Exchange Server 5.5 computer does not support 8-bit MIME formats.

To resolve this issue, disable 8-bit MIME for the Microsoft Windows SMTP service. For additional information about how to disable 8-bit MIME, click the article number below to view the article in the Microsoft Knowledge Base:

If you track one of the messages that is in the queue, the message gets submitted to the categorizer, but it goes no further. Also, if you increase the diagnostic logging level for MSExchangeTransport, the following event message is logged: If you restart the server with the SMTP service, the messages get delivered.