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Re: Are people trying to relay mail through my system?

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On 2023-09-25, Greg Wooledge <greg@wooledge.org> wrote:
>
> The preferred policy nowadays is to perform all possible checks *during*
> the initial SMTP conversation.  If a message fails to meet acceptance
> criteria for any reason, it should be rejected during that initial
> conversation.  Generating a bounce message almost always ends up sending
> spam to an innocent third party address, which the malicious sender has
> forged.
>
> How this relates to fetchmail and exim, specifically, I can't say.  These
> aren't tools I'm deeply familiar with.  But if you can do it, try to
> arrange it so that any message that can't be accepted gets dropped into
> a black hole, rather than generating a bounce message.
>
>

I guess this is what you're alluding to?

https://starcat.dp.ua/doc/exim4/FAQ-html/FAQ_7.html

 7. POLICY CONTROLS

 Q0701:  How do I block unwanted messages from outside my host?

 A0701:  Exim uses Access Control Lists (ACLs) for controlling incoming mail
 from other hosts. A whole chapter in the reference manual is devoted to
 describing how they work. A wide variety of conditions can be imposed on
 incoming messages.

 The default Exim run time configuration contains an example of an ACL which
 blocks all relaying, and messages whose senders cannot be verified. This
 example is heavily commented and worth studying.

 Q0702:  I don't want to block spam entirely; how can I inspect each message
 before deciding whether or not to deliver it?

 A0702:  Wherever possible, inspection and rejection is best done automatically
 in an ACL, that is, before the message is accepted. If you want to verify
 manually each message that is classified as spam by an automatic check, you can
 arrange for a system filter to freeze such messages after they have been
 accepted.

 If, after inspection, you decide not to deliver the message, it is safest to
 discard it, using the -Mrm option. Use of the -Mg option to force a bounce
 carries the risk of “collateral spam” if the sender address is faked (as it
 usually is in spam). 
 

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Thread

Are people trying to relay mail through my system? Rick Macdonald <rickmacd@shaw.ca> - 2023-09-25 05:00 +0200
  Re: Are people trying to relay mail through my system? Michael Kjörling <2695bd53d63c@ewoof.net> - 2023-09-25 16:40 +0200
    Re: Are people trying to relay mail through my system? Rick Macdonald <rickmacd@shaw.ca> - 2023-09-25 20:40 +0200
  Re: Are people trying to relay mail through my system? Andy Smith <andy@strugglers.net> - 2023-09-25 18:10 +0200
    Re: Are people trying to relay mail through my system? Rick Macdonald <rickmacd@shaw.ca> - 2023-09-25 20:30 +0200
      Re: Are people trying to relay mail through my system? Michael Kjörling <2695bd53d63c@ewoof.net> - 2023-09-25 20:50 +0200
        Re: Are people trying to relay mail through my system? Rick Macdonald <rickmacd@shaw.ca> - 2023-09-25 22:40 +0200
      Re: Are people trying to relay mail through my system? Andy Smith <andy@strugglers.net> - 2023-09-25 22:30 +0200
      Re: Are people trying to relay mail through my system? Andy Smith <andy@strugglers.net> - 2023-09-25 22:30 +0200
        Re: Are people trying to relay mail through my system? Rick Macdonald <rickmacd@shaw.ca> - 2023-09-25 23:00 +0200
          Re: Are people trying to relay mail through my system? Rick Macdonald <rickmacd@shaw.ca> - 2023-09-26 01:00 +0200
            Re: Are people trying to relay mail through my system? Greg Wooledge <greg@wooledge.org> - 2023-09-26 01:20 +0200
              Re: Are people trying to relay mail through my system? Rick Macdonald <rickmacd@shaw.ca> - 2023-09-26 03:50 +0200
              Re: Are people trying to relay mail through my system? Curt <curty@free.fr> - 2023-09-26 16:40 +0200

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