I'm currently experimenting with the Greylist feature.
Since I activated this feature spam seems to be gone completely.
However greylisting has some disadvantages if you're trying to receive mails from big hosters like GMX, Yahoo, Hotmail or EBay. Those hosters use a lot of different mailing servers to get their mails through to you. That's where the problems start.
Every time they try to send their mail, only the IP of the current mail server is whitelisted. The next time they try to send their mail it's probably another IP and mail isn't accepted again. If you're out of luck, the mail never gets through to you at all.
You can solve these problems by whitelisting all those servers so greylisting won't be used for that servers, but that's really a fulltime job as far as I can tell. Too many servers, too many changes in IP ranges...
So what if...
My idea: You've already implemented the feature of checking SPF entries and all big hosters already use SPF.
Could you give us an option in the greylisting menu to skip greylisting if the mail is coming from a SPF proven IP address?
That way we wouldn't have to deal with manual whitelisting and could still use greylisting without the fear of losing important mails.
Thanks for reading
Keep up the good work, best mail server ever!