I've just set up an SMTP server on a relatively little used domain using Postfix and enabled greylisting with SQLGrey. So far it seems to be working OK, and while there's the slight irritation of delays to emails from new senders, I can see from the logs that it's deterring a number of spam messages.

In your experience does greylisting effectively stop much spam? Is it a useful addition to e.g. SpamAssassin or is adding it on top overkill/unnecessary?

If I were to roll this out to heavier use domains (perhaps with more demanding users) would you anticipate a significant portion of poorly configured mail servers that would end up bouncing or losing messages?