XFOR: Should Have Exclusive Access to Port 25

• Microsoft Exchange Server, version 4.0

SYMPTOMS

The Microsoft Exchange Internet Mail Connector (IMC) requires exclusive use of TCP port 25 to successfully transport SMTP mail. If another program reserves port 25, the IMC may start without error, but no incoming mail will be accepted.

CAUSE

The Internet Mail Connector uses TCP port 25 for inbound SMTP mail as defined by RFC-821. For inbound SMTP mail, the IMC monitors port 25 for incoming connections from other SMTP hosts. The IMC will answer all incoming SMTP calls if it is the first or only SMTP service running.

If another SMTP service is running and set to receive mail, the Internet Mail Connector will start without error, but incoming mail will not be delivered to the Information Store. Incoming SMTP mail may be delivered to the third party program's delivery queue, or it may be rejected and returnedto the originating host.

The IMC may be able to deliver outbound mail even if another SMTP host is monitoring port 25 for incoming mail. Multiple outbound connections are possible, and when outbound mail is to be delivered, the IMC opens one or more connections to port 25 to connect to other SMTP hosts.

RESOLUTION

Do not run third party SMTP applications on any Microsoft Exchange server that is running the Internet Mail Connector service. You can verify if you have another SMTP service running by using the Windows NT telnet program.

To test the connection, run telnet from the Windows NT command prompt specifying the IP address of the server running the Internet Mail Connector, for example:

  telnet  xxx.45.66.77 25 

   220 imc.company.com service ready