OK So I installed Postfix and created a user called JudoOpsec. I sent an email to him at my domain and it didn't' bounce.
I sent another one to asdf at my domain and it bounced.
So I think everything is working.
When I try to use my email client to connect I use mail.mydomain.com which is my mx record for imap. But I don't know what to do for smtp. Do I create another mx record?