A while ago there was a long and emotional discussion on the qmail list
about whether a 5xx should be treated as permanent for the message or
permanent or one server that is in the MX list of the addressed domain.
This originates from the fact that some ISPs don't accept email from
dialup address space and signal a 5xx (permanent) to the sending MTA,
however they mean "we do not accept this message from that IP - try your
ISPs relay SMTP server".
Would it be a good idea/is there any interest in having a kinda BCP
document to get these clarified?
yes, it would be a good idea.
IMHO there's a difference between the meaning of a 5xx response from
an authoritative SMTP server for a recipient domain, and the meaning
of a 5xx response from some random other SMTP server.
but it might also be the case that some 5xx codes should be inherently
assumed to mean "per server" and others assumed to mean "per domain".