North American Network Operators Group

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Re: question on ptr rr

  • From: Suresh Ramasubramanian
  • Date: Sat Feb 07 06:38:13 2004

[email protected]  [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 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) // gpg : EDEDEFB9
manager, security and antispam operations