If domain.com mail is not handled by the server you connected to, then obviously the server is not going to accept your RCPT TO. This is basic anti-spam relay configuration.
I looked in /var/log/mail. Nothing has been written to it since early October.
I think I’m close now… I set up two accounts ‘accounts’ and ‘postmaster’. I can send mail to accounts@mydomain.com from another host via telnet. It’s not being delivered locally though. If I send mail locally using the ‘mail’ command from the ‘postmaster’ account to the ‘accounts’ account, they don’t get it. Oddly, mail sent from postmaster to root works, root gets the mail…