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Re: [ietf-smtp] Obsoleting RFC5321 552 -> 452 workaround

2019-12-16 21:57:37
On 12/16/19 10:30 PM, Viktor Dukhovni wrote:

This work-around was briefly trialed in Postfix about ten years back and
quickly abandoned.  It led to substantial volumes of mail clogging the
queue until all ~5 days of retries expire.  The reason being that not
all 552 errors at "RCPT TO" are in fact "too many recipients" errors,
and indeed given that servers are required to send "452", the majority
of these were hard mailbox quota errors or similar.  Treating these
as 452 and retrying was harmful to both sending and receiving systems.

I therefore seek community consensus (is there a WG for this, is this
list sufficient, ....?) to drop the work-around from 5321bis.  Servers
have now had plenty of time (over a decade) to switch from 552 to 442
on too many recipients.

I think it would make sense for 5321bis to

a) restate that a genuine "too many recipients" error (i.e. too many RCPT TO commands issued, or perhaps too many RCPT TO commands with distinct recipient addresses issued) MUST be reported via a 452 error code.

b) remove the recommendation that 552 in response to RCPT TO be treated as a per-recipient temporary error

c) [maybe] state that 552 has in past versions of the SMTP specification been used to indicate "too many recipients", but has also unfortunately been used to indicate other errors which should not be treated as temporary per-recipient errors.   So if a client implementation wishes to treat 552 as a per-recipient temporary error, it should perhaps limit the number and/or frequency of attempts to resend that message to that recipient.

(IMO, even "mailbox quota exceeded" should be treated as a temporary error, but the retry frequency should be reduced to something like once per day.   Some quota exceeded errors really are brief in nature, but some users simply stop reading their mail for an extended period of time.)


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