Here is a good website to test and make sure your DNS records are set correctly:

https://mxtoolbox.com/

A good way to test an email server on windows is to install telnet then connect to the mail server you want to test

telnet fabozzi.net 25

then type

ehlo fabozzi.net

and you should get a response press Ctrl+c then type quit to exit

On Linux runt he following command:

curl fabozzi.net:25

This will then return a response for the server if connection is successful