Many major internet service providers (ISPs) block port 25, the channel used for communication between an email client and an email server, to prevent their internet connection from being used for spam. When an email client that runs on your computer delivers messages, it uses port 25 to transmit data to remote email servers; therefore, if your ISP is blocking the port, your messages will not get through.
In order to work around this issue, we have opened Port 26. The ISPs that block port 25 require their SMTP server to be used instead of the remote SMTP server; so you will need to change the outgoing mail server (SMTP) from the default Port 25 to Port 26.
Some ISPs that block port 25 include:
- People PC
- Comcast ATTBI
If your ISP is on this list or you believe port 25 is blocked, you will need to reconfigure the account for your email client to change the outgoing SMTP port from 25 to 26 or 587.
What If Both Port 25, Port 26 and Port 587 Don't Work?
Some ISPs block both ports, and you will need to follow your ISP's procedures for sending email. You will typically have one or two choices. Sometimes you may contact your ISP to request that they open port 25. They may do this for you, although many ISPs have begun declining to do so. Some ISPs may require you to use their SMTP servers for all outgoing mail so that they can monitor outgoing email on their network in order to prevent spam.You may still send email from your domain (i.e. email@example.com); it just needs to be routed through their SMTP servers.
You'll want to contact your ISP about which SMTP servers you should use for outgoing email. This information is usually posted on a page of their website. If they have a search box on their site, searching for "port 25" will usually lead you to the relevant information.
My Mail Used to Work
Sometimes ISPs change their policies and start implementing blocks to certain ports without informing their customers (other than perhaps a post on their website). Most ISPs post information about Port 25 on their website if they have implemented a policy of blocking it. If sending mail suddenly stops working, this is probably why.