Peter J. Holzer writes:
I would rather state that some servers might still implement the error
from RFC 821 (do we have any data whether such servers exist at all and
how common they are) and that client may choose to accomodate them, but
should not generally treat 552 as a temporary errors.
I never did, and ~25 years ago it was pretty clear that all 4xx are
temporary, and all 5xx are permanent. It was always clear to me that the
remaining two digits are to be ignored, for the purposes of determining how
to handle the response.
I do not remember anyone ever raising the issue that I should've deferred a
552 error instead of bouncing it.
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