Dear Wietse,
thanks for your careful review.
On 2024-01-05 16:11:56 +0100, Wietse Venema via Postfix-users wrote:
Peter Wienemann via Postfix-users:
smtp(8):
------------------------------------------------------------------------
The Postfix SMTP+LMTP client supports multiple destinations separated
by comma or whitespace (Postfix 3.5 and later). Delivery is tried in the
specified order. For each individual destination the delivery rules of
RFC 5321, Section 5.1 are applied.
------------------------------------------------------------------------
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."
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.
and it ignores the possibility that SRV record lookups have been
enabled. I will keep the text that delivery is tried in the specified
order.
Yes, this is a valid point which I missed. Thanks for bringing this up.
transport(5):
------------------------------------------------------------------------
This tries to deliver to bar.example (following the rules of RFC 5321,
Section 5.1) before trying to deliver to foo.example.
------------------------------------------------------------------------
Here I think that the original text is more clear than the updated
text, which again implies that RFC 5321 applies to LMTP,
In this case delivery via LMTP is explicitly excluded since this refers
to the example:
example.com smtp:bar.example, foo.example
and ignores
the possibility of SRV record lookups or [] MX overrides.
Yes, SRV record lookups would still be an option. [] MX overrides are
incompatible with the given example.
Clearly, it is not desirable to enumerate all the SMTP client's
email delivery strategies here. Those strategies are orthogonal to
the issue of multiple destinations, and they do not belong here.
I like your comment. This gives rise to another option: Just refer to
the delivery strategy described in smtp(8). This would solve all the
problems mentioned above.
Although after re-reading smtp(8) in the given context, I think that the
cases
a) There are no MX records
b) MX lookup is switched off
c) SRV lookups are enabled
are not covered in the "description" section either. At least cases b)
and c) can be figured out by looking through subsequent sections. Case
a) is also missing in the explanation of the "domainname:port" syntax in
section "SMTP destination syntax".
Best regards,
Peter
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