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 26 as SMTP port is highly recommended.


Here’s how to, depending on your OS, check whether your ISP blocks port 25:


  • 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 25


  • Check port 25 in Mac OS

Each Mac OS is delivered with a preinstalled terminal emulator. The terminal is a tool which allows you to execute commands from the command line. Usually, the terminal is located under your Mac HD -> “Applications” directory -> Utilities sub-directory. When the window opens, you will see a command line with a flashing cursor on it so you can start typing right away. Just type the following command and press “Enter”:

telnet 25


  • Check port 25 in Linux


Open your favorite terminal emulator and execute the following command in order to check the connection on port 25 to your website:

telnet 25



Results from the 25 port check
If the test is successful, you should receive a message similar to this one: ESMTP Exim 4.63 #1 Wed, 18 Dec 2011 07:17:30 -0500
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.

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, Sibername offer an alternative SMTP port for the convenience of their customers.


The alternative SMTP port all Sibername customers can use is 26.


Information on how to change the SMTP port for the most popular mail clients can be found in our Email tutorial.

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