@laemeur "Callout verification failed" is an error to do with a server anti-spam protocol being too aggressive. Can't remember if it's SPF or Domain Keys or what, but it's almost certainly a badly configured mail server at the recipient or a relay server between your mail server and the recipient. And the recipient mail admin will almost certainly blame the sender mail admin and suggest that that the sender has to fix it by adding SPF records (or whatever) to his DNS entries. https://en.wikipedia.org/wiki/Callback_verification