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, 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 - those who have a need to send emails 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.
ISPs that block Port 25
This list contains some of the major ISPs that block port 25 on their servers:
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