Well, if you forward the message, some mail servers along the way have a habit of modifying the emails so after you parse it second time, the invalid TNEF part might have already been removed. That's most probably why you only get the error at the original server.
I checked your dlls and they are version 5584 (e.g. 2015 R3.1).
There has been some more enhancements when processing the TNEF emails since then.
So first of all, could you please try updating to the latest version of Rebex Secure Mail and if you still get the "Invalid TNEF message" exception with it on the original server, then please download the message as is via the GetMessage method:
and then zip it and send it back to us for analysis to firstname.lastname@example.org