Slightly OT: Postfix smtpd restrictions

Jason Ede J.Ede at
Fri Jun 25 10:06:47 IST 2010

I was thinking of sticking with just the unknown_reverse_client_hostname rather than the stricter one. As far as I can gather from the docs only if the IP doesn't resolve to a hostname will the email be rejected.


From: mailscanner-bounces at [mailto:mailscanner-bounces at] On Behalf Of Drew Marshall
Sent: 25 June 2010 09:02
To: MailScanner discussion
Subject: Re: Slightly OT: Postfix smtpd restrictions

On 24 Jun 2010, at 12:57, Jason Ede wrote:

This is purely MTA based, but since a lot of users here run postfix...

How have others found reject_unknown_reverse_client_hostname and the more harsh reject_unknown_client_hostname  in postfix? I'm debating implementing them here and wonder if others have found them problematic or useful? I thinking they should be good for weeding out spam emails and I can't see that they should catch legitimate senders, but want to be sure

I have to be honest, I ran this for a short while in warn mode and found that the number of people with mis-configured PTR records is huge (In the UK). For example, BT won't let anyone change their PTR, so EHLO/ HELO is always going to be a mismatch for these. Most people running Exchange 'naked' to the Internet end up having a EHLO as <name>.local or some such other non existent TLD so that ends up not matching either and so it goes on.

As ever YMMV but for me, I found other ways to combat spam coming from these sorts of connections such as RBLs etc.


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