It's also a problem with IP firewall whitelists. If the MX for a
domain is publicly advertised, but the MTA is behind a firewall with a
whitelist, then the firewall will get whacks of TCP SYNs from
spammers. And the rate at which the SYNs arrive will drop, once the
firewall rules are removed.
In other words, any attempt to protect yourself from spam will
result in the spammers attacking you even more, in the hope that
*something* will get past your filters. This increase in spam volume
means that the filters become less effective...
I'm still surprised that you manage to conserve much in the way
of resources by accepting these connections and blackholing the messages.
What happens if the messages are rejected hard
during the transaction - are they still retried? I'm sure I remember
postings about this in the early days of the list but I can't find the
numbers.
What do the figures look like? If you don't mind me asking.
--
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