On Mon 09/Oct/2023 08:23:33 +0200 Robert Mueller via mailop wrote:
I see that current setup might be useful in case some user changes MX
before the domain is activated at Fastmail, in which case giving 4xx
could make sense. But it is not right to report such re-tries to sender
score as attempts to deliver to non-existing users.
Yes, this is why we 4xx rather than 5xx email sent to an unconfigured domain.
Many inexperienced email users aren't sure exactly what order to change or set
things up in and we've seen users lose email before because they changed the MX
records before adding their domain at Fastmail. The 4xx response reduces the
chance of lost email.
That's bad.
First, nobody sends an important mail to a new domain without double checking
its delivery. It is very reasonable to loose a few test messages when you set
a new domain up.
By 4xx any unconfigured domain, a warning for delayed email can take hours and
several attempts. A 5xx would result in an immediate DSN, which is much better
in case of mistyping.
Best
Ale
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