Your other email client are not working correctly if they will allow that. Serious security breach that is. What client? so I can follow it up with the relevant vendor?

I send you encrypted mail because I want the information secured and encrypted between me and those I have shared my digital signature with. Allowing you to decide to forward my encrypted mail unencrypted to a third party defeats the whole point of encryption and represents a significant security hole.

By forwarding an s/mime attachment, if the third party is entitled to read my encrypted mail they will have the digital signature and therefore will be able to read the content. If they do not have the correct digital signature then they have no right to access the email. A simple way to defeat security breaches through lack of thought on the part of the parties to the communication.

Every single version of Outlook I have ever used allows forwarding of encrypted emails in clear text. Good luck trying to get Microsoft to change that :-)

In any event I found a way to circumvent the problem. Instead of Forward, if I click Reply the original message is in clear text and I can just change the To address to whomever I want.

If this is indeed a deliberate security feature of TB, they should have a way to disable it.