Hello,
Heiko Schlittermann writes:
> . verification of reverse DNS¹
> . helo verification¹
> . strict sender/recipient verification²
> . greylisting²
> . content scan²
> . delays
Also dynamic IP address filtering, and rather then using DROP or DENY, I
have found DEFER does a very nice job in frustrating multi-attempt spammers.
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DynaStop: Stopping one dynamic IP address at a time. Tested against 350+
million IP addresses!
http://tanaya.net/DynaStop/