Summary:

The following information, although tailored to EMF, applies to all SMTP servers.

All SMTP servers that send or receive Internet email should listen and send mail on TCP port 25. This however is not the only port that will be used by these servers to transfer SMTP mail.

Detailed Information:

Windows, or more appropriately, the TCP socket, will determine a high level dynamically-assigned port number across which to transfer data. This port is called an ephemeral port, and is defined as a port number higher then 1023. Ports lower then 1024, such as port 25, are called Well Known ports. The SMTP software running at the application layer of the OSI model has no control over the ephemeral port to be used by the TCP socket. The range of ephemeral ports used by TCP, when installed on Windows, can be configured in the Windows registry, however. The default is a range from 1024 to 5000.

What this means is that the EMF server will initially establish a connection with a remote SMTP server on port 25. The TCP socket will then dynamically select a port above 1023 over which the remainder of the TCP session will take place.

A FIREWALL MUST NOT BLOCK PORTS ABOVE 1023 OR SMTP WILL NOT WORK.

To visually see this on your EMF server:

• Open a command prompt on any workstation and manually telnet to your EMF server on port 25. Type:
  C:\> telnet server_name 25
  
  
• Open a command prompt on the EMF server and type:
  C:\> netstat -a -p tcp

  Proto  Local Address          Foreign Address        State



  TCP    langley:1260           bast.tumbleweed.com:smtp  ESTABLISHED

 

Additional Information:

A list of Well Known ports can be found at:

http://www.iana.org/assignments/port-numbers