> Although I could write some scripts to poll the user db at the mail server > back ends. It would be nice if I could be somewhat lazy have have a plugin > do this. ;) Real time checking for each delivery is obviously unworkable, > but poll then cache with maybe a expiry is still a practical option. The > mechanics are probably similar to greylisting.
We're doing real-time checking. It certainly has a performance hit. We tried caching but the hit rate was incredibly low -- the overwhelmingly vast majority of addresses you'll get are randomly made-up invalid addresses -- so it wasn't worth the extra code and codepaths and we ripped it out with no noticeable difference. I might be able to show you some code but I'd need to get permission, and it's not really very good code anyway ;) -Jared
