At 03:25 PM 27-12-2019, John C Klensin wrote:
Probably worth reviewing RFC 1846 to see what it says about
these issues. I have not done that yet and it isn't at the top
of my queue.
RFC 1846 sets a requirement with two possible paths:
1. Retry another (MX) host.
2. Give up and return a NDR.
Was there some past discussion to use 421 for (1) instead of a 521?
RFC 1846 proposed a minimal smtp server. The minimal implementation
defined in RFC 5321 would not be compliant  with RFC 1846 and RFC
7504 if that implementation does not support the postmaster address.
1. Section 4.5.1 of RFC 5321 states that the postmaster address is
strictly not necessary if the server always returns a 554.
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