[Djigzo users] "Message could not be encrypted"

Good afternoon, I've the following problem: An external user is configured for mandatory encryption (to receive only encrypted emails). We have different users in our organization he wants to communicate with, some of them encrypted, some unencrypted (unencrypted only in incoming direction!). If that external user send's an unencrypted email to one of our internal users, that email get blocked with an error "The message with Subject <Subject> has not been sent to the following recipients because the message could not be encrypted" According to the documentation, the locality should determine the direction of encryption, it seems that this doe not work as intended. Is there any configuration setting to prevent that ? regards Christian