Problem

Is my ISP blocking port 25?
How to find out whether port 25 is blocked by your ISP?

Solution

This article explains how to find out whether your ISP is blocking port 25 on your account.

TCP Port 25 blocking prevents any 3rd party outgoing mail servers from being accessed while connected to your ISP's (Internet Service Provider) network.

Many ISP's (Internet Service Providers) are blocking what is called "Port 25" which is the port used to send emails. They have chosen to do this to cut down on the amount of "SPAM" mail that is sent. Some ISPs have gone one step further and restrict the sending of emails through their servers by using their domain name only. I.e. blocking the sending of emails using your own domain name.

Unfortunately there is nothing you can do to change this. The only solution is to change your outbound SMTP server to a alternative SMTP provider. outMail is such a service that solves this problem and enables you to send outbound emails from any device, from any location using any ISP. For those ISPs that block, filter or redirect port 25; outMail provides alternative SMTP ports as well (25, 587, 2525 and 8025)

Testing if port 25 is blocked

For the below example we are going to use a utility called "telnet". For windows platforms you may need to install this utility from "Windows Programs and Features"

telnet smtp.myisp.com 25

If port 25 is not blocked you should get a reply similar to the below

Trying x.x.x.x
Connected to smtp.myisp.com
Escape character is '^]'
220 smtp.myisp.com ESMTP Postfix

If port 25 is blocked you might get something like this

Trying x.x.x.x
Connected to smtp.myisp.com
Escape character is '^]'
Connection closed by foreign host

Remember if TCP port 25 is being filter or redirected you will still get a connect to the port, but your email may not send still. It is for that reason outMail provides alternative ports.

Below is what you would see if you telnet to an outMail server

Trying x.x.x.x
Connected to smtp.myisp.com
Escape character is '^]'
220 mxXXXX.smtp-engine.com ESMTP prolateral-outmail

If you see the above message "prolateral-outmail" then your port 25 isn't being blocked.