Why my ISP blocks SMTP port 25? What can I do?

Many ISPs and firewall configurations now block outgoing mail / SMTP connections on port 25 (the default port for SMTP) – this is mainly done to try and stop spam going out through their networks.

What is a port?

In order for two computers or devices on the Internet to communicate they have to follow a set of communication rules, this is called a protocol, most things on the Internet use the TCP/IP set or stack of protocols.

Each of these protocols is assigned a unique port number to communicate over so that the computers always know exactly what protocol they need to use, to communicate.Some commonly used protocols are:

• HTTP – used for websites and uses port 80
• DNS – used for resolving Internet domain names to IP addresses and uses port 53
• FTP – used for transferring files across the Internet and uses port 21
• POP3 – used for collecting email and uses port 110
• IMAP – also used for collecting email and uses port 143

What is port 25 and why is it blocked?

Port 25 is the default port used to transport or communicate email across the Internet using the SMTP protocol.

Many ISPs and broadband / cable providers now block or restrict SMTP connections on port 25 (the default port for SMTP ), the main reason for this is because when a computer gets infected by a virus it can be hijacked by the virus writers to send out thousands, if not millions of spam emails – the ISPs have to prevent this from happening and blocking the port is the only real solution.

Why does this affect me?

Home / Small Business Users

The issue can cause problems when you need to bypass their servers(their SMTP servers may be unreliable, blacklisted or overly restrictive) or if they block port 25 and do not provide an SMTP server themselves for you to send emails out.

How can I check if my ISP blocks ports

You can run telnet smtp.forwardemail.net 25 from command line or terminal to see if your outbound port 25 connection is blocked.

Workaround ISP blocking inbound SMTP on port 25

The standard value of the SMTP port is 25, and internet providers generally block this value. If this is your case, then sending and receiving email seems impossible. Solve the problem with the following two methods:

• Change the value of SMTP to 26
• Select 587 after switching to the TLS port.