Where, and how, are you encrypting the messages with the ZIX encryption?
If you are using ZIX at the desktop, you need to make certain you are not modifying either the SENT FROM or REPLY TO e-mail addresses in the message header.
If you are using the ZIX gateway, then you need to, again, make certain that the SENDING ADDRESS is a valid address on your SmarterMail server, that you have not modified the SENT FROM or REPLY TO e-mail addresses, in the header, and that your message is PROPERLY AUTHENTICATED by the SENDER's e-mail address.
A failure in any of these areas will cause non delivery, even if the messages are able to get through the outbound SmarterMail message queue as all e-mail must now be fully authenticated back to the SENDING E-MAIL ADDRESS.
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