Thank you for sending the mail to us. We investigated into it and the 'Invalid TNEF message' exception is caused by the fact that the TNEF ('winmail.dat') contains another embedded TNEF message which contains another one. We never encountered this before and this is not a supported feature yet.
We will try to solve this problem in one of the future releases (not the next one, which is already quite near).
Update: Support for TNEF-within-TNEF messages was added in Rebex Secure Mail 2012 R3!