Mail relay is properly configured in Plesk. Domains on the server can send email just fine but to emails to SOME of them bounce back with an rcpthosts error.

Kind of rcpthosts in reverse issue, lol. In rcpthosts, the domains in question are missing. Why would Plesk not write these domains to that file at creation? Should I just add these domains that are missing?

Now have this error coming in on multiple servers....and my admins seem to be sleeping.

Okay, we found out what is happening and it is very bad. When you use the backup and restore utilities to migrate an account from one server to another, it appears to bring the rcpthosts file with it and overwrites the one on the server!! It also overwrites /var/qmail/control/me.

Talk about bug of all bugs!

The fix is to generate a new rcpthosts file. This will do it:

ls -1 /home/httpd/vhosts |grep -v "default" |grep -v "webmail" > /var/qmail/control/rcpthosts