Issues arise where mail sent to big ISPs is blocked (i.e.  From Windows 2003 server or a box with telnet installed, open a command prompt.
Test to see what MX record is viewed by other external mail servers (i.e. 
From command prompt on your MAIL SERVER:
C:Documents and SettingsAdministrator>nslookup
Default Server:
> set q=mx
Non-authoritative answer:     MX preference = 5, mail exchanger =     MX preference = 5, mail exchanger =     nameserver =     nameserver =     nameserver = internet address = internet address =      internet address =      internet address =      internet address =
Open another command prompt and type:
[telnet IP OF COMCAST 25 (port 25, smtp)]
telnet 25
Here are the results:
220 comcast ESMTP server ready
501 EHLO requires valid address
501 HELO requires valid address
Then type "ehlo"
The output will be the publicly viewable MX record that the comcast mail server is communicating with.
Even if you control your own public DNS and your DNS registrar is pointing to your servers, this only takes care of forward lookup zones.  You need to contact your ISP for reverse lookup zones (PTR records).