Not accepting messages on port 25

So I just migrated a bunch of accounts to the CentOS Deep Castle VM image.  (Previously running on Windows)

If anyone attempts to send on port 25 they get a 501 permission denied after the MAIL FROM command.

501 5.7.1 <adam@plexicomm.net>... Permission denied

The submission port 587 works, but there are still plenty of devices out there configured for port 25.  For the moment I worked around this with a NAT rule.  I can't figure out what settings are causing this.  Any insights?