When we send any email from a cpanel server with "Verify the existance of email senders" turned on in the exim configuration in WHM to any email account on our other server at highforge.com we get the following immediate bounce error:

550-Unrouteable address
550 Sender verify failed

We are using the default nameservers 1and1 assigned us so I'm not sure what the problem might be. Records in Plesk for highforge.com look clean, but maybe I'm missing something. If it's RDNS related, what specifically would we need to tell the registrar? Any other advice or suggestions?

A SMTP dialogue to your highforge.com server looks pretty good:

> telnet highforge.com 25
220 u15151460.onlinehome-server.com ESMTP
mail from: a (at) b.com
250 ok
rcpt to: a (at) highforge.com
250 ok
So I doubt it's a Plesk problem.