[Mimedefang] SNARE spam detection

[Paul Murphy]

>> Both operations look potentially expensive, and port-scanning the 
>> sender means all our legitimate senders will soon see regular port 
>> scans.

>Yeah, the port-scanning looks troublesome, especially if you do it in
real-time.

In many cases (and both of the networks I run), that'll get you firewalled
immediately.  Port scans are not friendly actions...

Proper implementation of SPF or a similar system across all mail domains
would cut spamming by 99% overnight, and would remove almost all of the risk
from phishing mails.  Don't re-invent the wheel - just get the design from
last year to be accepted by more people.

Paul.