Tech Support Pits:

From Cory===
Dear Webby,
What's reverse resolution all about ?
Cory

Dear Cory
Reverse DNS tracking is a way to locate a legitimate server,
or to determine if a mail is probably from an illegitimate
server, like for example from a spammer.

Reverse DNS backtracks not from you, but from the root of
the Internet as far as possible to the sender of a mail.
If it arrives at the sender, and identifies the sending server,
then the mail is usually legit. If the trace stops or runs out,
then the IP number uses as the apparent origin of the mail
Has been forged, or the sending server is not 100% legit.

Some shady ISPs sell unused capacity to spammers to pay
for their dope bills, and naturally they don't want reverse
tracing of spam to point straight at them. For that reason
they don't set up their servers to be found with reverse
resolution.

That fact is quite commonly used for blocking spam.
If a sending server has no reverse resolution, then that
server is automatically blacklisted and mail from it winds
up in the trash. It's the equivalent of slamming the phone
on people who have paid extra to have their name or number
hidden from a phone's call display.

If you want to check the reverse DNS resolution of any e-mail,
view the header of that e-mail and look for the originating
IP number, copy it, and paste it into the REV DNS field at
http://www.dnsstuff.com/

Have FUN
Dear Webby