Hi,
----- Transcript of session follows ----- 553 5.3.5 mail.domainname.org. config error: mail loops back to me (MX problem?)
Sendmail doesn't know it is supposed to accept mail for this domain. You should add the domainname and any subdomains you receive mail for in class w of sendmail. Either /etc/sendmail.cw or GUI --> Control Panel --> Email server (or anything similar; can't remember).
The DNS and MX configuration seems to be the same as all other sites on the server.
It doesn't appear to be an MX problem. Good luck... Nico