Check everything works well

To make sure your mail server works as expected, you can send an email to friends. however, this leads to issues :

You must wait for their answer :
You won't get every data you may need ;
You need friends 😜.

Luckily there are robots taht can automatically answer. As example :

echo@generic-nic.net
echo@nic.fr
autoreply@dmarctest.org
https://dkimvalidator.com/results website

Take a look at mxtoolbox and mail-tester too, the last one is very nice.