MX records should point to FQDN not IP addresses, so make sure that isn't an issue. You can check to see if your mail is backed up by:
du -h /var/qmail/queue/local |more
or
du -h /var/qmail/queue/mess|more
Assuming everyone is happy and mail is being delivered correctly you should see only 4.0K per folder, any folder over that has contents and should be investigated. I would check your MX record, most smtp daemons will not deliver to an MX record pointing to an IP address.
Hope this helped,
Nathan