I'm curious as to how many other hosting companies are doing a sender callout for all incoming mail into their servers? Callouts would verify the validity of the envelop-sender of an incoming message. We are starting to get a few complaints regarding the callout system. Personally, I think it is a good system that blocks a lot of spam, but obviously if the majority of users are complaining, then the system is not very effective. I'm not really anywhere near having the majority of users complain, but I have gotten enough complaints to raise some questions. So I'm just curious how many other hosting companies utilize sender callouts? If you do use sender callouts, how do you explain to users that complain about the system interfering with mail coming from mail servers that either do not adhere to standards or are sending out e-mail messages from invalid e-mail addresses?