Hannah Schroeter wrote:
So 2821 seems to regard usage of 251/551 as quite optional
and sometimes even advised against.
Yes, when I mentioned STD 10 251/551 it was because I consider
the simplified 2821-forwarding to 3rd parties as FUBAR. It's
not a question of the result code and privacy, 2xx instead of
251 is fine. But it's the responsibility of this hop, and if
it's unable to take this responsibility (read: bounces), then
551 can be more useful for all parties than "5xx user unknown",
let alone "5xx infradead hates SPF" ;-)
Bye, Frank