Of course you can, have a look at:
http://slett.net/spam-filtering-for-mx/collateral.html#signedsender
and:
http://slett.net/spam-filtering-for-mx/exim-sign.html
What I do is using signed envelope sender and accepting bounces for those ones. Bounces addressed to real users but not signed are also accepted but they are just eaten by a blackhole acl.
I think Tony Finch has also done an implementation, but I cannot find back the exact url.
On Tue, 28 Jun 2005 09:23:52 +0100
exim-users-request@??? wrote:
> On 27 Jun 2005 at 18:07, Mark Smith wrote about
> "RE: [exim] Supressing bounce messag":
>
> |...
> | (I wrote:)
> | > OK, then how about addressing the problem at its source? You
> | > shouldn't be generating bounces to external senders in the
> | > first place. Undeliverable mail should be rejected at SMTP time.
> |
> | We don't generate bounces to external senders, only local senders, and we
> | only do that because some MUAs don't cope well with rejections at SMTP time.
>
> Right. Good.
>
> | The situation I'm trying to address is the unlikely event that someone on
> | our system (a local user or a hacker) starts Joe-Jobbing somebody. We don't
> | want to send misdirected bounces to the addresses that are being spoofed.
>
> But if all the hacker's victim addresses are valid, that's OK? ;)
>
> Can you prohibit spoofed senders (at least spoofed external senders)
> instead?
>
> - Fred
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