Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321

Sam Varshavchik <mrsam@courier-mta.com> Sun, 27 September 2020 12:58 UTC

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From: Sam Varshavchik <mrsam@courier-mta.com>
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Subject: Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321
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Keith Moore writes:

> 3. 821, 1123, and subsequent revisions all seem to be based on the  
> assumption that if you're operating an SMTP server, you're trying in good  
> faith to deliver (legitimate) email reliably.   I'm not sure this assumption

And you should have a pretty good idea of what your IP address is.
 
> Seen from that perspective, maybe 5321's language about EHLO arguments could  
> use some updating along the following lines:
>
> - For a very many reasons [which could be listed, or not], SMTP servers have  
> no reasonable expectation of being able to determine the validity or  
> legitimacy of a message based on comparison of the EHLO command argument  
> with anything else at all.   Therefore if what you're trying to do is  
> reliably deliver legitimate mail (for some meaning of legitimate),  
> validation of EHLO arguments is useless and strongly NOT RECOMMENDED.

The exact phraseology is only secondary. The point I was making is that I  
see that EHLO/HELO validation is employed in practice, and it is in  
practical use. And based on my own experience, it is highly effective. Like  
I said, in 20+ years I've been doing strict domain validation on HELO/EHLO I  
do not recall a single false positive, and a mind-boggling amount of crap  
that got blocked.

And I think that in practical situations this is going to outrank, in  
peoples' minds, any demand that they MUST NOT do that.

> Of course, if your goal is really to discard mail for no good reason, and  
> you're not handling incoming mail for anyone but yourself, have at it!    
> Just have the decency to blackhole the mail rather than bounce it, since  
> you're really not doing anyone any favors.

On that point I'll also have to disagree. It's better to reject the mail  
with a 5xx, than /dev/null it.