Some of you may be aware of a mail-server practice called "greylisting". It works roughly like this: you send an email, the mail server at the other end says "not now, wait a bit", your mail server tries again in a few minutes and then the mail is delivered. The idea behind this is that spammers who have hijacked other people's computers for the purposes of sending out crap will not have bothered with niceties like retrying failed deliveries and will just push on to the next piece of floating junk.
I encountered today a company who provide mail filtering services for their customers (it was one of their customers who had called me) who take this one step further. Instead of using a 'soft' refusal -- "not now, try again later" -- they actually bounce the first piece of mail you send their customer -- "not known at this address" -- and then let any more sent from the same address through.
'Course, by the time one message has been bounced, the person at the other end -- if it IS a person -- assumes that the email address doesn't work, and doesn't try it again. Or, alternatively, they have to waste time getting in contact with the person they were trying to email by some other means.
Not exactly the way I'd choose to have my mail handled....