Why can I not send mail through port 25?

Port 25 Blocking

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.

Our solution

HTMLgraphic uses another port to get around the blocking of port 25. If your a hosting customer we allow all a work around all you need to do is use port 2525 instead of port 25.

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 Htmlgraphic Designs mail servers:

Incoming Connections

HTMLgraphic Designs mail servers use port 25 to accept incoming connections from e-mail clients. You can freely change that value in both the server, 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

HTMLgraphic Designs mail servers also use the port 25 for sending. It connects to remote servers and delivers the messages from your mail client. 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 can still send email messages (use port 2525 on your outgoing connections).

ISPs that block Port 25

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

AT&T	MindSpring
BellSouth	MSN
CableOne	NetZero
Charter	People PC
Comcast ATTBI	Sprynet
Cox	Sympatico.ca
EarthLink	Verio
Flashnet	Verizon
MediaOne