Re: [Asrg] What are the IPs that sends mail for a domain?

Alessandro Vesely <vesely@tana.it> Wed, 17 June 2009 13:32 UTC

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Date: Wed, 17 Jun 2009 15:33:08 +0200
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Subject: Re: [Asrg] What are the IPs that sends mail for a domain?
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Bill Cole wrote:
> Lyndon Nerenberg wrote, On 6/16/09 9:55 PM:
>> On Tue, 2009-06-16 at 17:24 -0700, Douglas Otis wrote:
>>> IMHO, all outbound MTAs should be required to return CVS records for
>>> their EHLO name and offer MX records for their inbound.
>>
>> Doug, are you sure that's what you meant to say? The sentence is a bit
>> ambiguous. Are you really saying any host that sends mail (is an SMTP
>> client) MUST also host an listed SMTP server?
> 
> I can't testify to what he meant, but I think what he is actually saying 
> is that if you have a machine that says "EHLO some.name" then there 
> should be both a MX record for some.name and a SRV record for 
> _client._smtp.some.name (i.e. a CSV/CSA record).

However, the standard requires that it says "EHLO host-at.some.name". 
It is a seemingly simple task to drop the leftmost label(s) so as to 
obtain the mail domain, but doing that properly requires a zone cut 
algorithm that most servers miss.

> That doesn't mean requiring inbound SMTP on every outbound, it means 
> requiring an affirmation in DNS that a name can be used in EHLO by a 
> particular IP address and a way to get mail to the responsible party for 
> the machine(s) using that name in EHLO. This is an admirable goal. A 
> weaker goal would be to get people running non-spamming mail servers to 
> follow the existing accepted standard of using a valid resolvable FQDN 
> in EHLO.

If we have a weaker goal and an admirable one, we're better off if 
they don't conflict with each other. We cannot ask for a domain name 
after EHLO, except for tiny ESPs whose domain name, host name, and IP 
address are the same thing. If we need the domain name, we can either 
mandate the zone cut algorithm, or use a different verb than EHLO.