I need to set up a mailserver. I've several different domains and I need to set up different emails for each other.
domain1.com -> firstname.lastname@example.org
domain2.com -> email@example.com
While waiting for more details, a very good point to start with could be the relevant webpages from the email server the user is running.
The whole thing mainly boils down to configure local users to be mapped to specific email address(es).
Without any extra details, that's it.