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Port 25 Blocking

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Many ISPs are blocking what is called "Port 25" which is the port used to send e-mail. They are doing this to cut down on the amount of spam that is sent from their networks.

All e-mail sent via the Internet is routed through the port 25, the channel used for communication between an e-mail client and an e-mail server. Even though port 25 blocking will probably become an industry standard, however, the filter can create problems for e-mail servers and block legitimate e-mail as well as spam.

Port 25 blocking allows ISPs to block spam sent out through their networks, but it tends to punish the innocent that have a need to send through e-mail servers other than those belonging to their ISP. The ISPs that block port 25 require their SMTP server to be used instead of the remote SMTP server or a SMTP server running on your computer.

How the port 25 is used

All e-mail sent via the Internet is routed through port 25. When an e-mail server that runs on your computer delivers messages, it always uses port 25 to transmit data to remote e-mail servers. Therefore, if your ISP is blocking the port, your messages will not get through. There are two different ways the port 25 is being used by PostCast Server:

Incoming Connections

PostCast Server uses port 25 to accept incoming connections from e-mail clients. You can freely change that value in both server and client program and everything will continue to work because all TCP/IP connections are directed to your computer. Unless you block connections to your computer, the program will accept messages using any port number you specify (1-65535).

Outgoing Connections

PostCast Server also uses the port 25 for sending. It connects to remote servers and delivers the messages from the Outbox folder. Exactly the same rules apply except that every remote server expects the connection ONLY on port #25. This is the standard port number and while you can change the port number in the program to allow clients to send the messages internally, the remote servers always use port 25. If your ISP blocks remote connections to port 25, you cannot send any messages. PostCast Server will not be able to connect to the remote servers.

ISPs that block Port 25

This list contains some of the major ISPs that block port 25 on their servers:

AT&T (can be unblocked at the request)







People PC

Comcast ATTBI









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You can detect whether your ISP blocks port 25 using the Setup Wizard in PostCast Server. In the Network Diagnostics step, press the "Port 25 blocking" button to run the test:

You can also see if the port is blocked by running a telnet command:

Press Start/Run and enter:


Replace [emailserver] with the address of any external e-mail server. For example:

Do not use your ISP's e-mail server address. If the port is not blocked, you should receive a response starting with the '220 ' string. 



To bypass the port 25 blocking you have these options:

Use a different ISP

You can use a different ISP to connect to the Internet. Smaller local ISPs usually do not block Port 25. Here are some web sites that can help you find thousands of ISPs:

Use socks proxy servers

You can send e-mail using the socks proxy access to a computer on the Internet. This feature enables you to relay e-mail through other servers. When the message is sent using a third-party socks proxy, your IP address does not appear as the source of the message.

The best solution is to connect to your ISP's socks proxy directly if it is provided by the ISP. Some ISPs offer access to their socks proxy server. See if your ISP provides socks proxy access in the support section on their web site. If they do, you can use their socks proxy server address to configure the program to send messages. Their server's (non-dynamic) IP address will be the source of your outgoing messages instead of your dynamic IP address assigned to your computer at the moment your Internet connection is established.

Use backup SMTP servers

You can specify one or more backup SMTP servers and instruct the program to forward all messages to them. This is not a complete solution because the program will still be unable to send messages from your computer. For more information, see SMTP Gateways.

If you do not need to send messages

If you only want to receive messages sent to the server you can use the "Mail Reflector" service offered by This service enables them to be the primary e-mail exchanger for your domain. When e-mail destined for your domain arrives at their servers, they forward it on to your inbound e-mail server, which can be on a different (and unblocked) port of your choosing. Price is $39.95 per Year. See this web page for more information:

Anti-Spam Methods:

  1. Overview
  2. Port 25 Blocking
  3. Internet Black and White Lists
  4. Blocking Open Relays
  5. DNS Lookups
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