Quite nice so far and doing most things I've expected very well

Unfortunately in some situations I get a User unknown in relay recipient table and I've no idea how to solve this.
In general my mail-flow is:
internet -> efa -> decryption/encryption-box (postfix) -> ms-exchange (2013)
For recipient validation I've put a relay_recipient_maps = ldap:/etc/postfix/ldap_relay_recipient_map.cf in main.cf which works very well.
For some special cases I have an other mailflow:
internet -> efa -> decryption/encryption-box (postfix) -> mailbox-server (postfix)
What do I have to change to get the recipient addresses validated against the mailbox-server (or at least against the decryption-box which asks the mailbox-server)?
Even if I leave the decryption-box out of the traffic like: internet -> efa -> mailbox-server (postfix) I get the User unknown in relay recipient table.
Any help is welcome
regards
Robert