Exactly as explained (even with example commands!) in topic http://www.tunnelbroker.net/forums/index.php?topic=304.0
Professional: Verify that your MTA has working reverse DNS (ex: dig mx $domain +short ; dig aaaa $mx +short ; dig -x $mxAAAA +short)
So we look at the hostname/domain provided in the email address you submitted, and then run basically the equivalent those commands.
If you have problems, then you should perhaps email email@example.com
with details of your problem.
what exactly do you check for the "Professional"-test?
the official MX(-PTR) of the domain the user provided or the PTR of the mailserver entered in the "Administrator"-test?
Because both don't seem to work for me.