Hi Mike,

That's answers the second part of the question I initially asked, where the reverse and forward domains don't exactly match for any of the reasons you mention below.

What I am not sure about is who's problem is that? Is it mine on the receiving end, or theirs? If they are using mismatched hosts is that not something they are doing in a non standard way, and if so why should I open myself up to more spam to accommodate them?

I'm not exactly a networking person so I don't know, just asking to learn..

Thanks again,


On 19/09/2016 17:43, Mike Tubby wrote:

On 9/19/2016 4:29 PM, Dave Lugo wrote:
On Mon, 19 Sep 2016, Mike Tubby wrote:

There is no 'law' that says your reverse DNS must work and its simply dangerous to use the heuristic no rDNS => High probability of SPAM.

I respectfully disagree.  It's as dangerous as any other very effective
spam filtering method - high accuracy, low FPs.

Yes, you should have some way to override the missing rDNS check. But
rejecting on missing rDNS is mostly safe, in my opinion and experience.

My point is that there's nothing in any of the RFCs that says your reverse DNS must work which is why we perform our checking against known block lists such as SpamHaus et. al.

Our experience is that rDNS cannot be used reliably for several reasons that include:

    * multiple hosts behind load balancer

* mis-match between exact host and generic host like "mx01a.megacorp.com" and "mx.megacorp.com"

* internal hosts calling out through firewalls, eg. host MSEXCH01.internal.megacorp.com calls out through a firewall with a public IP that either reverses to "fw.megacorp.com" or in case of some organisations like the police is simply anonymous (no rDNS)

hence our experience is that it is dangerous to attribute lack of correct rDNS to being SPAM, however YMMV ;-)


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