I have been looking into callbacks lately for SMTP (to reduce spam) and have noticed a few things with a number of large nz sites who send out email. A number of sites send from email addresses which don't exist. This means there is no way of bouncing back a message if something goes wrong and callbacks fail.. Large site such as trademe.co.nz,ferrit.co.nz,pxt.vodafone.co.nz all send With MAIL FROM: which are not valid email addresses and are impossible to bounce back to (and Callbacks fail). There are a few other small ones I've noticed.. but overall callbacks work fine. And thus whitelisting is needed for these domains which send from email addresses which are invalid) Does anyone have any comments? Suggestions? Why callbacks are the devil (or otherwise)? Why callbacks work really well..Anyone use using them? who they had to whitelist if they used them in NZ? Note: also noted there are a number of mail servers in nz which don't accept bounces at all (even to valid users) :-( rfc-ignorant.com anyone? Thanks Craig