Postfix can't receive, SMTP client-server loop detected

Ubuntu 12.4, mail bounces with "SMTP client-server loop detected".

Although this server at one time handled the email of some six domains, I've since let them all expire. I've left the server as it was, e.g., I can still get the home page of the first of the domains when I link to it via the server's address.

I'd like to send email to the server, to root@hostname. I can ping the hostname. mail reports root has no new mail. My hostname is entered properly (I think) in main.cf, as mydestination = name_of.the_host.com, localhost.the_host.com, localhost Would you please guide me to how to resolve this or where to begin to look? I'm largely ignorant of Postfix, however.

Thanks.