How to check whether FTP port 21 is blocked?

The default TCP port for FTP connection is port 21. Sometimes the local Internet Service Provider blocks this port and this will result in FTP connection issues.
Here's how to, depending on your OS, check whether your ISP blocks port 21:

Check port 21 with a Windows OS
Click on the Start menu button, click "Run".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 21 Check port 21 with Apple/Mac
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 21