Geert Hendrickx via Postfix-users:
> On Tue, Oct 21, 2025 at 08:17:41 -0400, Wietse Venema via Postfix-users wrote:
> > Geert Hendrickx via Postfix-users:
> > > You don't have to lookup *all* of them, you can stop as soon as one valid
> > > MX
> > > record is found, so the vast majority of cases would only need a single
> > > extra
> > > lookup.
> >
> > And when there is no valid MX record, you'd have to look up all of them?
>
> Yes, just like you have to for e-mail delivery.
I suppose that was a trick question.
The answer is "you don't". The SMTP standard (RFC 5322) does not
require that an SMTP client tries every MX host.
For fairness, the Postfix SMTP client limits the effort that Postfix
will spend on a delivery request, with smtp_mx_address_limit and
smtp_mx_session_limit.
Additionally, the Postfix DNS library enforces a response size limit
of 100 records, which limits the effort that the SMTP server will
spend on check_xxx_{a,mx,ns}_access and reject_unknown_xxx_domain.
Wietse
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