My ISP blocks SMTP on 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.

Mail Servers

If you have setup a mail server using software such as Sendmail, Postfix or Microsoft Exchange on your Internet connection and port 25 is blocked you will not be able to send or receive external emails.

How can AuthSMTP help?

AuthSMTP can solve this problem for you - we operate AuthSMTP SMTP servers on the following ports:

  • 23
  • 25 (Not usually recommended)
  • 26
  • 2525

AuthSMTP also has many other advantages such as:

  • Can be used from almost any Internet connection including Wi-Fi or 3G connections
  • Can be used on almost any of your computers or devices including your iPhone, iPad, smart phone, laptop or tablet
  • If you have multiple email accounts, they can all be configured to send using the service
  • Quick and easy to setup - send email within minutes.
  • Full setup guides for most major programs and devices.
  • Outbound virus scanning and (optional) email duplication.

How do I use AuthSMTP?

If you do not already have an AuthSMTP account you can view the pricing plans and sign-up on our pricing page - the base account is only $32 / £20 / €24 per year and you can sign-up and start sending email within minutes.

For details on how to setup your email program to use AuthSMTP with alternate SMTP ports click here.

What if it doesn't work?

If it does not work (or your email program does not support changing the SMTP port) contact your system administrator / ISP about allowing outbound connections to our servers on ports: 23, 25, 26 or 2525.

In the event our service does not solve your problem and you cannot send email we do offer a money back guarantee.