It's the second time I have this kind of problem with a customer. The message is:
5.1.0 - Unknown address error 550-'Callback setup failed while verifying <[email protected].\n(result of an earlier callout reused).\nThe initial connection, or a HELO or MAIL FROM:<> command was\nrejected. Refusing MAIL FROM:<> does not help fight spam, disregards\nRFC requirements, and stops you from receiving standard bounce\nmessages. This host does not accept mail from domains whose servers\nrefuse bounces.\nUser Not Found!' (delivery attempts: 0)
Some one heard about it ?