XIMS: Using Internet Mail Service Mail: "503 Sender Already Specified"

Occasionally, messages from Exchange Server through the Internet Mail Service (Internet Mail Connector in version 4.0) to a remote SMTP e-mail domain are rejected. A non-delivery report (NDR) is returned to the sender with the following error message:

Exchange Server makes connections to remote SMTP mailers or relay hosts to deliver SMTP mail. If more than one message is queued to a remote domain, Exchange Server can send multiple messages during a single connection. If the remote SMTP server is not RFC-compliant, it improperly rejects the delivery of a second message from the Internet Mail Service during the course of a single connection. Then it sends the NDR mentioned above.

The sequence of events is as follows:

• Exchange Server's Internet Mail Service finishes the first message and receives a "250 OK" acknowledgement that the message was accepted.
• The Internet Mail Service then attempts to send the second message in this same connection by sending the MAIL FROM command.
• The remote host improperly rejects the message with the "503 Sender already specified" error message. This is not RFC 821 compliant according to section 4.1.1.

Although this is a problem with the remote system, you can correct the problem by setting the Internet Mail Service to send only one message for each connection. This is a global setting that affects the performance of the Internet Mail Service because it no longer sends multiple messages during a single connection to any remote domain. This may be a minor issue for some e-mail networks and a preferable option to the above problem.