[Exim] Hosts which retry when given 5xx at end of data (v3 -…

Greetings,

Recalling this snippet from v3 spec.txt chapter 45 :

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Unfortunately, there are a number of mailers in use that treat any SMTP error
response given after the data has been transmitted as a temporary failure.
Exim sends code 550 when it rejects a message because of a bad sender, and RFC
821 is quite clear in stating that all codes starting with 5 are always
'permanent negative completion' replies. However, it does not give any
guidance as to what should be done on receiving such replies, and some mailers
persist in trying to send messages when they receive such a code at the end of
the data.

[...]

In an attempt to shut such MTAs up, if the same host sends the same bad sender
for a third time within 24 hours, MAIL is accepted, but all subsequent RCPT
commands are rejected with a 550 error code. This means 'unknown user' and if
a remote mailer doesn't treat that as a hard error, it is very seriously
broken.

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We encountered just this problem recently - but with exim v4. As it
happens, the sender attempted to reach us with the traditional
mass-mail "bad header",

To: <Undisclosed-Recipient:;>

and we attempted to reject it on syntax. But I guess this would be
the same for any kind of rejection at end of data?

Anyhow, this resulted in the sending host subjecting us to days of
repeated retries, until I manually blacklisted the envelope sender to
get rid of it.

Any thoughts of a solution that works in v4, following the kind of
strategy as described for v3 above?

all the best