How to troubleshoot mail flow issues related to MX records and DNS

Issues arise where mail sent to big ISPs is blocked (i.e. Comcast.net).  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. comcast.net). 
From command prompt on your MAIL SERVER:
 
C:Documents and SettingsAdministrator>nslookup
Default Server:  ns1.lax.pnap.net
Address:  216.52.254.1
> set q=mx
> comcast.net
Server:  ns1.lax.pnap.net
Address:  216.52.254.1
Non-authoritative answer:
comcast.net     MX preference = 5, mail exchanger = mx2.comcast.net
comcast.net     MX preference = 5, mail exchanger = mx1.comcast.net
comcast.net     nameserver = dns101.comcast.net
comcast.net     nameserver = dns102.comcast.net
comcast.net     nameserver = dns103.comcast.net
mx1.comcast.net internet address = 76.96.62.116
mx2.comcast.net internet address = 76.96.30.116
dns101.cmcast.net      internet address = 68.87.64.204
dns102.comcast.net      internet address = 68.87.66.204
dns103.comcast.net      internet address = 68.87.76.228
>
 
Open another command prompt and type:
 
[telnet IP OF COMCAST 25 (port 25, smtp)]
 
telnet 76.96.62.116 25
 
Here are the results:
 
220 IMTA26.westchester.pa.mail.comcast.net comcast ESMTP server ready
ehlo
501 EHLO requires valid address
helo
501 HELO requires valid address
 
Then type "ehlo yourdomain.com"
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).

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