And, here's the results. You can, I'm sure, immediately pick out the point where I turned on the greylisting service. It's not a complete solution - I still get some spam - but you can see from the graph that I'm getting around 1/3 as much inbound mail as I was getting before.
It's even more pronounced if you look at the month view
Notice that it affects the sent, as well as received, because so much of my outbound email was reject and bounce messages.
hi Rich --
quick note -- you really shouldn't be generating reject and bounce messages in response to spam!
Nowadays, the generation of bounces in response to spam is considered bad form -- it's called "backscatter", and due to changing spammer tactics, it's become a big problem, worthy of blocking in its own right. We offer an anti-backscatter ruleset in SpamAssassin, for example. Some sites (e.g. Spamcop, iirc) even tend to blacklist MTAs that send a lot of backscatter.
Here's more info on this:
http://www.spamresource.com/2007/02/backscatter-what-is-it-how-do-i-stop-it.html
anyway, you should really try to ensure no responses are sent from your machine in response to spam or viruses, except via 5XX status messages in the SMTP transaction.