I am onto a strange problem after I upgraded first the CentOS (i386) working server to CentOS v5.6 in order to patch the Apache Killer, and then the mailserver was down. After much efforts and tweaks, it works, but now it can send mails but not receiving. A portion of the /var/log/maillog points something trying to connect to example.com which should not exist in the first place and I could not figure out it's existence even in mysql:
For me this looks like an outgoing mail, that shall be send to any box in at examle.com. You could check your mailqueue with the mailq command. Then search the mailid in your mail spool folder, check it's content for any further steps where the mail could've come from.
