On 26/02/2016 14:42, Matej Sustr wrote:
Hello,
I have recently gotten myself into an argument with a vendor about
RFC5321 and what constitutes a successful "delivery attempt", in a
specific case when TCP/IP connection to primary MX succeeds but e-mail
delivery fails either with a 4yz code or a timeout.
There is a slight ambiguity between RFC5321 section 5.1 (and also
RFC2821 section 5) which says:
... To provide reliable
mail transmission, the SMTP client MUST be able to try (and retry)
each of the relevant addresses in this list [MX->A/AAAA] in order,
until a
delivery attempt succeeds.
It says 'MUST *be able to* try', not 'MUST try'.
It continues to say:
"However, there MAY also be a configurable limit on the number of
alternate addresses that can be tried. In any case, the SMTP client
SHOULD try at least two addresses."
So, just trying one address is allowed (but not encouraged) by the standard.
There are arguments both ways.
If I get a 4xx response (eg due to greylisting) then it is entirely
possible (maybe likely) that I get it from all MX servers, so trying all
of the servers just adds work for no benefit.
If I can't connect to one server, then I should almost certainly try the
next one.
If I get a 5xx response, then should I try the other servers or not? By
your reading of the standard you should, but I'd say probably not. (But
a 5xx could potentially be 'this server's configuration is totally
broken' rather than 'that recipient is unrecognised')
In the case of 4xx or 5xx there are reasons you can think of why you
should or shouldn't try the other servers.
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