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From "David F. Skoll" <...@roaringpenguin.com>
Subject Re: Rejecting without backscatter (was Re: Spamassassin not catching spam (Follow-up))
Date Thu, 26 Mar 2015 15:52:05 GMT
On Thu, 26 Mar 2015 10:12:22 -0500 (CDT)
Dave Funk <dbfunk@engineering.uiowa.edu> wrote:

> If they are compatible you respond with a 250, if not with a 452 (or
> other 45* type reply).

We looked at doing this.  There are some serious downsides:

1) Some senders (for example, mailing list tools) send to quite a number
of recipients at once.  30 or even 100 is not out of the question.
If all of them have different policies, the last recipient is going to
wait a very long time indeed to receive his or her email.

2) Some marginal SMTP software (old versions of Novell Groupwise, I
think?  Can't recall exactly) does not handle 4xx responses to RCPT:
very well.  It basically converts them to 5xx.

3) You have no control over the retry interval or retention time on the
SMTP client.  It's not unimaginable that some messages simply won't get
delivered because the SMTP client gives up.  Some SMTP clients use
an exponential backoff algorithm rather than a constant retry interval,
and that can be disastrous in this situation.

> Note that Gmail is already doing something like this (the "multiple
> destinations not supported in one transaction" status).

You can possibly get away with it on a per-domain rather than
per-recipient basis because you're unlikely to have a single message
coming in for more than a handful of different domains.  Even so, it's
risky IMO.

Regards,

David.

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