On Fri, Jan 05, 2024 at 06:46:01PM +0100, Peter Wienemann via Postfix-users
wrote:
> > Unfortunately this says that RFC 5321 applies to LMTP deliveries,
>
> RFC 2033 says: "The LMTP protocol is identical to the SMTP protocol [SMTP]
> [HOST-REQ] with its service extensions [ESMTP], except as modified by this
> document."
But, in fact LMTP MX records are only for SMTP relay, and to not apply
to either submission or LMTP.
> I do not find any exceptions mentioned in RFC 2033 concerning the choice of
> target hosts. Therefore - to my understanding - the same target selection
> rules apply for SMTP and LMTP. If this is a misunderstanding on my part,
> please correct me.
They do not. MX records specify the location of the inbound SMTP
servers for a domain, and there is no reason to expect to find the
associated LMTP servers at the same set of servers.
In any case, LMTP delivery is a local matter (replacing ad-hoc local
delivery IPC mechanisms), and there is no reason for a sending domain
domain to connect to the LMTP servers of an unrelated domain, so the
resolution mechanism is unspecified.
Indeed, how exactly would you specify unix-domain sockets in MX records?
This is why LMTP nexthops are just transport end-points, not logical
destinations subject to MX lookup.
Since the RFC failed to note this distinction, the problem is with the RFC.
--
Viktor.
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