I used a hosting provider who had this turned on for a while and it often had odd behavior. There are a ton of email servers that are not configured properly or

I used a hosting provider who had this turned on for a while and it often had odd behavior. There are a ton of email servers that are not configured properly or are configured in complex ways. The worst case I saw was with a large corporate email system. They used some kind of round robin/load balaner. Incoming mail went through one mailserver, outgoing through multiple (different) mailservers. The greylisting would therefore always get caught in an infinite loop. Here's a rough example: 1) The greylisting would initially reject the mail from bigcorp.com. 2) Bigcorp.com would get the rejection, but the re-send would come from a different mail server: mailserver1.bigcorp.com. 3) Since this is a new IP address, the greylisting would bounce the new email. 4) Bigcorp.com would re-send, but this time from mailserver8.bigcorp.com. 5) rinse and repeat Now, you can say that the guys at bigcorp.com are boneheads and they should hide these details from the outside world. But the reality is, as an end-user, I don't care. I just want to get my mail.