Re: [exim] How multi-recipient messages are handled?

On Sunday, 18 November 2018 11:50:54 AM AEDT Aki Kyo via Exim-users wrote:
> Hi, I have a question about handling messages with multiple recipients in
> Exim.
>
> I want to run something like rspamd and apply per-user spam filtering,
> which means multiple recipients might have different spam scoring
> results or different white/black list results on the same message. I
> know it depends when it's done, like during the SMTP process or after
> accept.
>
> I use Postfix now and I can force it to reply with 452 4.5.3 error
> ("too many recipients") if there are 2 or more recipients so rspamd
> milter will never have multiple recipient messages but this is bad
> behavior and frowned upon I believe.

As Jasen and Viktor mentioned with PRDR and it likely being deployed almost
nowhere you are left with deferring recipients, but it doesn't have to be
limited to 1 per message, merely one recipient type per message. Content
filtering can only be done at the end of receiving the whole message, at which
point smtp without PRDR only allows a per message signal of accept or reject.
If however you have reason to believe that a group of your recipients would
differ on the outcome then you can accept each group at a time (e.g. a spam
lovers group, a strict group and a normal group. If you see your first
recipient from the spam lovers group then defer all strict or normal
recipients but allow additional spam lovers). This probably wouldn't work down
to individual bayes databases and the like, but could handle per user
whitelists/blacklists (as the envelope sender is already available at rcpt to
time) and a common "score" threshold.

>
> How does Exim handle this situation, what is the recommended approach
> taken by Exim administrators? I appreciate links, opinions, tips.
> Thank you.
>
> Aki