I have the forward and reverse dns records. The domain is <rcan_dot_nl>

The problem is indeed about bounced emails. However the receiving ispmailserver validates the mailsender by verifying the sender, in this case me_at_rcan_dot_nl, BY querying the Astaro, being the smtpproxy. The smtpproxy does not know about me_at_rcan_dot_nl and the message is then rejected. I think the Astaro should query the inside mailserver.

As you can see white-listing will not do.