I'm not sure exactly whats going on, but I appear to have a problem receiving e-mails from certain domains. The domains in question are on 2 servers, one of which is also hosted at ThePlanet. I have disabled my firewall and installed a packet sniffer, but see no incoming connections from the servers in question. E-mail from everywhere else works just fine and I can actually send to these servers, but emails sent from them just seem to disappear. Since the remote server doesn't even attempt to connect, I can't see how it would be related to my mailserver. I don't have access to log files from the remote servers, so I don't know what else to try. Any suggestions?

Are those machines using a nameserver that's serving them stale info? See what those machines think the MX records are for your domain.

-Chris

you might wanna check the rbls to see if your ip is on a blacklist.. you can also call up theplanet and ask them to troubleshoot.. see if they have your ip in a list as well.

As far as I can tell, the MX records look to be correct for both. I did a nslookup on one and a "domain lookup" through cpanel on the other.

I checked my ip using http://www.robtex.com/rbl/ and it doesn't appear to be blacklisted anywhere.

I have it working now. It was actually a stale MX record. Thanks everyone for all the help.