How to check whether SMTP port 25 is blocked?
The standard port used for SMTP connections is 25. One of the most common reasons for not being able to send messages is that your Internet service provider blocks this port. This is why using an alternative SMTP port is highly recommended.
Check port 25 in Windows
Click on the Start menu button, usually located at the lower left corner of the screen. There is a tab called "Run". Click it and a new window will pop up. In the text field of this window you need to type cmd and then press the "Enter" key. Another window will open. It is the so called "MS-DOS prompt" or "command prompt". Just type the following command in the command prompt window and press "Enter":
telnet mail.myhostcp.com 25 <ENTER>
The results from the 25 port check
If the test is successful, you should receive a message similar to this one:
If you get a message saying that the connection is refused we recommend that you use the alternative port for your SMTP connection.
What is the alternative SMTP port and how to configure an email client to use it?
The reason why most ISPs block the default SMTP port 25 is because it is used for sending unsolicited emails.
Fortunately, web hosting providers such as Hostinguk offer an alternative SMTP port for the convenience of their customers.
The alternative SMTP port all Hostinguk customers can use is 587, changing the SMTP port from 25 to 587 will resolve the issue in the majority of cases.