In brief, their mailservers temporarily reject incoming mail from unknown senders / mail server combinations, relying on RFC2821 (SMTP) error codes to force the sending mailserver to retry 10 minutes later. If the spammer is using hijacked machines, this will most likely not happen; if the sender is a legitimate sender using a standards-compliant mail server, automagic kicks in and the email is delivered as it should (albeit 10 minutes delayed).
I like it.
In theory, this delay could be user configurable depending on the confidence one has in one's whitelist (list of accepted correspondents) thereby increasing the probability of the fraudulent use of a hijacked relaying mail server is discovered and closed down.
Anders Jacobsen |