ASSP utilises a feature called "greylisting" to combat spam - based on the idea that most spammers do not use RFC compliant mailers:

Greylisting is a new weapon to use against spam in this great war being waged upon it. With this new shielding method, by which you may block out huge amounts of spam, you are sure to please your email clients!

In name, as well as operation, greylisting is related to whitelisting and blacklisting. What happen is that each time a given mailbox receives an email from an unknown contact (ip), that mail is rejected with a "try again later"-message (This happens at the SMTP layer and is transparent to the end user). This, in the short run, means that all mail gets delayed at least until the sender tries again - but this is where spam loses out! Most spam is not sent out using RFC compliant MTAs; the spamming software will not try again later.

[From Greylisting.org - a great weapon against spammers]

It is initially annoying - as all emails get delayed until they are in the list of known "ok" originators, but on the other hand seems to work very well.

More about this when I have had a few weeks worth of data to assess.

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