[Dshield] DNS MX record block question

Kenneth Porter shiva at sewingwitch.com
Wed Sep 10 04:28:52 GMT 2003

--On Tuesday, September 09, 2003 3:37 PM +0200 security at admin.fulgan.com wrote:

> Why would you want to do that ?

[Probing envelope sender's domain's MX for active port 25.]

Just ran across this interesting paper on "greylisting" on the MIMEDefang list:


It uses tempfail to stop dumb spammers who don't retry on failure, at the cost
of an additional delivery delay for the first piece of legitimate mail between
a given sender and recipient. (Subsequent messages don't suffer the delay.)

The clever part is that it maintains a DB of triplets of envelope sender,
envelope recipient, and MTA IP. Mail with one such triplet is temporarily
refused the first time seen and allowed on all subsequent tries.

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