Re: Filtering on two headers / checking MX entries ?

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Corey Halpin wrote:
>   Also, does anybody know of a way to check if the origin of an email (as 
> determined by the earliest of the Received headers) is actually an MX?
>   I'd like to avoid sending confirm messages based on email from machines that 
> weren't MXes.

Doing this is likely a very bad idea - you will get far too many false
positives.

It's perfectly legal (and not uncommon) for sites to have different
systems (or even just IP addresses on the same systems) for inbound and
outbound mail processing.

However, take a look at SPF - http://www.openspf.org/ - which allows
domain owners to publish a list of valid outbound mail servers, so that
you can perform a very similar check to your original idea.
Unfortunately, SPF isn't as widely adopted as one would like.
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