I have checked the domain records, reboot the server, even deleted the record for the domain and re-entered it and I still get this error. Any ideas? It only happens on 1 domain. Do I have to delete the entire site and start over?

are you doing your own DNS?
i had the same problems, doing my own DNS.

i have

Virtual Sites >
ns1.mydomain.com > IP1
ns2.mydomain.com > IP2
www.mydomain.com > IP1
(with my user + catch all setup at www.mydomain.com)

DNS >
ns1.mydomain.com > IP1
ns2.mydomain.com > IP2
www.mydomain.com > IP1
domain.com > IP1

PTR >
ns1.mydomain.com > IP1
ns2.mydomain.com > IP2

MX
mydomain.com > high > mydomain.com
www.mydomain.com > high > mydomain.com


i searched around here a little bit, and saw a post with the same problem, and someone replied that his dns was getting confused with the domain name, since there were a few entries with the same domain name.

so i deleted my domain completely from the DNS settings, and re-entered my dns settings for ns1, ns2, & www exactly the way they were before, ONLY, this time, i set my MX to:

mydomain.com > high > www.mydomain.com
www.mydomain.com > high > www.mydomain.com


saved my dns settings. then saved the settings again at the CP screen(so the services would restart). waiting 15 or so minutes. then my catch-all and my aliases worked fine.


whew.
so yeah, that fixed my problem. i don't know if it will help with yours, but, i just finished working on this a few days ago, so i thought i'd relay my story. sorry if it doesn't help.

from the sun cobalt knowledgebase:

This error is generated when domain.com or the fully qualified domain name [i.e.: mail.domain.com]
is not entered into the Host/Domain Aliases field under Email Server Parameters.

This is corrected by adding a valid domain or fully qualified domain name inside the field.

I have an advice. Usually customers like to have mail.domain.com as their mail server and it often causes the mail loops back error.

So I setup the MX as:

www.mydomain.com > high > mail.mydomain.com

and save the DNS Settings. Then I go to the virtual site mydomain.com site admin and click on site settings. There I first check the receive email for domain and save settings and then I uncheck it and save it again.. and again I check it and save it..

That usually does it all.


-Omair

Hi! We had this exact problem... the trick is:
make sure your LOCAL-HOST-NAMES (or accept email for domains) box in your admin area has:
domain.com
www.domain.com
mail.domain.com

or anything else applicable and make sure that your RELAY MAIL FOR DOMAINS does NOT have any of those entries in it (domain.com www.domain.com etc...)