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

Laura Atkins <> Mon, 28 September 2020 13:49 UTC

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From: Laura Atkins <>
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Date: Mon, 28 Sep 2020 14:49:27 +0100
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Subject: Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321
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> On 28 Sep 2020, at 14:26, Keith Moore <> wrote:
> On 9/28/20 9:10 AM, Laura Atkins wrote:
>>> I've made those arguments multiple times already.  "evidence" seems like the wrong thing to ask for because this is really a question of design - what choices should be made to allow the email network to continue operating seamlessly and efficiently in the event of widespread use of NAT within the network (either to gateway between IPv4 and IPv6 or to economize use of IPv4 space)?
>> I’ve seen lots of arguments, yes, but I’ve not seen any real evidence backing up your assertions.
> Most of this is what I've picked up reading IETF lists and trade publications for years.   I guess I thought it was common knowledge within IETF that NATs were increasingly being used in these ways - certainly there were many discussions about it in IETF several years ago.   
NATs, yes. NATs handling significant amounts of SMTP traffic, I’ve not seen any evidence for.

And if it’s just an issue of NATs, there are other solutions. So the question really becomes: is this an issue affecting large amounts of email? What emails is it affecting? Is it likely to affect email in the future? That’s the kind of evidence I was asking for. Tell me, how much traffic, does this affect now? How much traffic do we expect this to affect moving forward? What tools are we removing from filters if we say they cannot evaulate the hostname? What impact does that have on them? 

The next round of questions will look something like:

Is this a RFC status that MTA operators are simply going to ignore? In the email space we have some very large players who handle billions of emails a day and blatantly violate the RFCs. ’But the RFC says’ is not, in my experience, a persuasive argument for these types. They usually have significant analysis and data that shows that the operational and human problems resulting from their lack of implementation is not enough to force them to follow the RFCs. Simply saying something MUST or MUST NOT be done doesn’t actually result in that happening. 

And then there is the abuse problem. I understand that it’s outside the scope of the rewrite here but it is relevant for adoption. There are absolutely cases where interoperability takes a back seat to protecting the network. 
>> I’m trying to better understand your point of view and understand why this is so important. But it’s been hard to find that in the sniping. 
>> The design question is one that should be discussed, but is this the correct space for that? 
> Arguably the discussion should be taking place on the emailcore WG mailing list rather than here.   But we started the discussion here.
>>> The changes I see happening include the increasing scarcity of IPv4 address space and the consequent emergence of IPv6-only network providers using NAT to move packets between IPv4 and IPv6 addressing domains.  I'm also anticipating the need to eventually phase out the public IPv4 Internet altogether.    
>> Or we could write the BIS to recommend anyone running v4 services also provide v6 services, couldn’t we? Take the burden off the v6 only systems which are likely newer entrants and put it on the ‘old timers’ who’ve been around for decades. 
> We could make such a recommendation.   Though I expect, that like spam filterers, network operators will do what they want, and consider themselves justified in doing so, regardless of what we recommend.
> I suspect that the task before us is to make recommendations that both spam filterers and network operators will find palatable.
So why are we wasting time on the EHLO evaulation question, if that’s the case. 

>>> (From operators' perspective: how long does it make sense for every network to maintain its IPv4 baggage, just so that email won't be blocked?   At the very least we need input from network operators.)
>> My experience is that folks running MTAs on IPv6 actually enact stricter technical requirements on those systems. For instance, many IPv4 servers will accept mail without valid rDNS on the sending IP or will accept mail without SPF records, while the IPv6 servers run by the same entities require those things directly. 
> It might even make sense for IPv6 operations to "raise the bar".   For example, PTR lookup of IPv6 addresses allows finer-grained delegation than PTR lookup of IPv4 addresses.   But regardless of what some operators are doing, I think this requires careful analysis to justify from a standards perspective.   I suspect there will be IPv4-only networks with a legitimate need to originate mail to IPv6-only servers.
You suspect, but I’m not convinced your suspicions are a reason to upend current practices. 


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