Craig Whitmore wrote:
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.
Callbacks can provide value, but can also be a royal pain-in-the-ass as well. The wikipedia page[1] has some comments on the pros/cons; but I have seen machines die from callback initiated DDoS/DoS. Not pretty... but I guess you take the risk with any anti-spam mechanism. [1] http://en.wikipedia.org/wiki/Callback_verification aj.