You are facing some issues in sending emails (e.g. when you try to send multiple mails with the same message), and you receive an error message "TNEF decoding of message failed". You are using a Kerio Connect version older than version 9.2.4.
A fix for this issue was released on version 9.2.4 resolving the issue with TNEF decoder corrupting the whole email if signed, as detailed in the Product Releases.
Please proceed to Upgrade Kerio Connect to the Latest Version, either automatically or manually.
Note for the upgrade to 9.3.1 p2 version: if the upgrade doesn't succeed, try upgrading to Kerio 9.3.1 first and then apply the patch 2 update.