> Ideally, a SMTP return code should differentiate the reason for rejection.
> There should be a different code for non-existing user, technical problems
> (like mailbox full) or policy-based reject.

You know, they actually did think of that.

https://datatracker.ietf.org/doc/html/rfc5321#section-4.2.3

https://datatracker.ietf.org/doc/html/rfc3463#page-14

> Although I'm not sure if in the
> latter case it is possible to differentiate if the policy rejection is
> "general" (eg. related to sending IP) or for the particular sender's email
> address.

Doesn't look like it to me.

-- 
Atro Tossavainen, Chairman of the Board
Infinite Mho Oy, Helsinki, Finland
tel. +358-44-5000 600, http://www.infinitemho.fi/
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