Strange Email

Post the output from the Terminal.app command postconf -n and somebody here can have a look at the current configuration. Alternatively, here is a discussion of some tools that can be used to detect an open relay.

Yes, you might be an open relay, and that would be bad. Or you might have a weak password, and the senders are using that to spam you. A compromised client is also possible.

Or somebody just set a bogus return address to match one of yours, and spammed your user. Folks do that. That would require no compromises, after all.

To determine where and how the mail message arrived, you'll need to look at the SMTP (and possibly POP or IMAP) logs. Console.app is good at that, as Server Admin.app doesn't have very much data available in its log view.

Blocking an IP address won't even slow the folks down, FWIW.

FWIW, "mydomain.com" is a real and registered domain, though I'll assume you were using that to obfuscate your domain; that you're not the registered owner of that domain. If you were obfuscating, please use example.com, example.org or example.net for that purpose. Those three are reserved for this sort of usage.