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 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 my-domain-name.com 25 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": How to check whether SMTP port 25 is blocked? 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 my-domain-name.com 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 my-domain-name.com 25 220-serv01.sitegroundXXX.com ESMTP Exim.