I would suggest you implement SSL for your ingoing and outgoing mail which means that you will no longer be using port 25. The default configuration file should have automatically opened the port and be listening. However, to answer your question you can see if the port is open and what process is listening with the following command:
Oct 19, 2018 How to check open ports in Linux using the CLI - nixCraft Jul 22, 2019 How to open ports in Windows Firewall | Windows Central Sep 22, 2017 Port Checker - Check Open Ports Online Port Checker is a simple and free online tool for checking open ports on your computer/device, often useful in testing port forwarding settings on a router. For instance, if you're facing connection issues with a program (email, IM client etc) then it may be possible that the port required by the application is getting blocked by your router's
Jul 03, 2017
In the External port and Internal port boxes, type the port number. Generally, this number is the same. Click either TCP or UDP, and then click OK. Repeat steps 1 through 9 for each port to open.----- To find open ports on a computer, use netstat command line. To display all open ports, open DOS command, type netstat and press Enter. How to Check if SMTP Port 587 is Not Blocked: A Step-by Jun 16, 2020
Port Forwarding: How to Check Your Router for Open Ports
How to check open ports in Linux using the CLI - nixCraft