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[Discuss] Strange sendmail (and postfix) spam issue: accepting fail "from" myself?

On Tue, Mar 29, 2016 at 02:09:15PM -0400, Derek Atkins wrote:
> So now I want to focus on Sendmail.  Any sendmail guru's out there?

Hello Derek!

Did you ever find a solution for this for Sendmail?  I would never
call myself a Sendmail guru, especially now--but I recall I spent some
time on trying to solve this a while back and failed.  It seems the
Sendmail folks believe that fixing this is a bad idea, because it can
block legitimate mail e.g. if someone at your site sends mail to
someone at another site that has a .forward file that points to an
address at your site.  In case that's not clear:

  From: foo at
  To: bar at

And bar at has a .forward file that forwards to
baz at

Apparently, this message will get lost.  This seems like it should be
a fixable problem, but pffft.

I did just find this recipe, which appears to be outdated:

I also thought SPF and/or DMARC would fix this, but I never got around
to trying to set any of that up...

Derek D. Martin   GPG Key ID: 0xDFBEAD02
This message is posted from an invalid address.  Replying to it will result in
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