[Mimedefang] Powerful anti-spam rule that never discards vali d e-mail

[local.mimedefang]

On Tue, 14 Jan 2003 14:29:48 -0000, dmuell at gmx.net (Dirk Mueller)
wrote:

>On Die, 14 Jan 2003, local.mimedefang wrote:
>
>> Hmm. I'm wondering how effective it would be to set up a mail
>> relay with a higher priority than the "real" ones which does nothing
>> but tempfail *everything*. It shouldn't be too hard to write but
>> I'm a bit loath to try it on a real domain, especially one which
>> belongs to my employer!
>
>I don't think its that simple. I've noticed several times spam software 
>injecting mails in the "fallback" mail servers, without ever having tried 
>the regular mx point. 

OK well we have some old domains which are no longer used and which
normally don't have MX records, but spammer's databases are full of
addresses in those domains.

So I put back two MX records, a priority 1 pointing at a test machine
and a priority 5 pointing at one of our normal mail gateways.

On the test machine I wrote a little SMTP server in Perl which goes
as far as the DATA phase before giving a 452.

I am getting *some* mail to the domain arriving at the lower priority
gateway but only a small fraction of those which hit the test 
gateway. I haven't counted yet though so I can't tell you what the
proportion is.

I sent two mails from my home machine to an address in the test
domain. Both were rejected by the test gateway and later arrived
at the real gateway. But there did seem to be a delay, which is a
bit worrying. More testing needed...