XIMS: Using Internet Mail Service Mail: "503 Sender Already Specified" (202765)

SYMPTOMS

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:

503 Sender already specified.

CAUSE

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.

WORKAROUND

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.