Yes, from what I can tell, iHug started doing this recently. Wait, no, they are checking that my mail server would accept messages for the envelope sender address. Because mail from a bunch of users' PHP code has an envelope sender of apache(a)hostname (but a valid "From:" header in each email), I have to accept mail for apache(a)hostname (or at least pretend I do). it's not really a /huge/ problem, but it's annoying to have to do so. Infact, I haven't bothered yet.
Yes callbacks..They are very helpfull and there are people on both sides which give good/bad reasons for them, but they definately do reduce the amount of spam coming in a lot, but you have to be carefull in whitelisting a few of them common problems like machines sending from www-data@ apache@ etc (there are quite a lot). IMHO if "something" sends out email. then there must be the possibility to "bounce" something back to it for whatever reason and sending the original message from an invalid (an email address which doesn't exist) MFROM doesn't help at all. Thanks Craig .