question on ptr rr

Sat Feb 7 11:33:05 UTC 2004

garrett.allen at comcast.net  [2/7/2004 4:55 PM] :
> this may be deemed off topic - if so apologies in advance. however i respect many of the opinions i see here so thought i would take a chance and ask. 
> 
> we are a stub network, injesting about 30k emails daily.  about a year ago we implemented a spam filtering product.  it works well.  recently we turned on the knob to enable it to do reverse lookups.  only the mild version, a reverse is made on the ptr rr for the ip address sending the email.  if it fails the spam filter issues a 421 and closes the connection.  unfortunately, we have 6 sites thus far that are legitimately trying to communicate with us but don't have ptr's associated with the ip address sending emails.  since it obviously isn't a requirement to have one is it generally accepted to do so?  any sense for how many end networks do and don't?

Having proper rDNS is a good thing, strongly recommended but definitely 
not required for sending mail.

There are quite a few sites (including the freebsd.org mailserver, and, 
on a case by case basis, even AOL) that do refuse mail from IPs without 
rDNS, but turning on a "must have rDNS or you can't email us" setting 
will definitely result in a non trivial amount of false positives.

-- 
srs (postmaster|suresh)@outblaze.com // gpg : EDEDEFB9
manager, outblaze.com security and antispam operations