How to Find an SMTP Port Number
Networks assign port numbers to different types of incoming and outgoing traffic. One example of this is the port assigned to SMTP (Simple Mail Transfer Protocol), which is used to package and transfer electronic mail, or email. In most cases, SMTP is assigned to port 25 in the network's Exchange server. If you want to find out which port your network uses to route SMTP traffic, you can check it by logging in to the Exchange server.
Log in to the Exchange System Manager in your mail server.
Find the icon labeled "Default SMTP Virtual Server" and right-click on the icon.
Select the "General" tab and click the button marked "Advanced."
Read the value listed in the entry for Transmission Control Protocol (TCP). This value is the SMTP port used by your network.