Why do Internet Providers Block SMTP Port 25?

A number of ISPs (Internet Service Providers) are making it a common practice to block Port 25, the TCP port used to send email. The reasoning behind this practice is to reduce the amount of junk mail being sent from their networks. Some of the providers that currently block port 25 include AT&T, Comcast, TDS, One Communications, Charter and MSN just to name a few. AT&T will however, remove the block at user request.

Every email sent over the internet passes through port 25, which acts as a communications channel between the mail client and mail server. While blocking this port may never become an industry standard, it is another effective method, especially when considering that spam filters can create problems for mail servers and also block legitimate mail. The major issue with port 25 blocking is that it tends to punish innocent users that have a need to send mail through servers other than those owned by their ISP. Providers that block port 25 call for you to use their SMTP server rather than a remote SMTP server or the mail server running on your computer.

How Port 25 Works

When the mail server that runs on your computer sends a message, it will always use port 25 to transfer data to remote mail servers. If an ISP is blocking this port, your messages have no way of getting through. The PostCast Server uses port 25 in two ways:

Incoming Connections - This method is used to accept incoming connections from mail clients. By changing the value in both the server and client, mail will continue to flow as all TCP/IP connections are routed to your computer. Unless connections to your computer are blocked, the client will accept messages using any port you specify from 1 to 65535.

Outgoing Connections - The PostCast Server also uses port 25 to send mail. This is done by connecting to remote servers and delivering messages from an outbox folder. The same rules apply with the exception that remote servers will only expect the connection on port 25.

Although the port number can be changed to allow clients to internally send messages, 25 is the standard port number and remote servers will always use it. Unfortunately, if your provider blocks remote connections to port 25, you will not be able to send any email as the PostCast Server has no way to establish a connection.

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