Also, keep in mind that "unbreakable" and "not possible to decrypt" are snake oil terms. More responsible approach requires stating clearly what is covered by your security model, and what isn't (see many great documents on TrueCrypt pages). That not even mentioning side-channel attacks.

Personally, I don't see much sense in such program, as encrypting things via any trusted cipher (with or without frontends/bundles, like TrueCrypt) and sending them as attachments *or* encrypted plain text, achieves just that (as long as receiver know password and have keyfiles, obviously).

You might aim for making it easier to use (i.e. user write a message, click "send", and software does all encrypting/sending, leaving tedious manual work apart), but that would be better achieved via frontend using actually existing tools - like, existing and already installed cipher packages and mail client, just acting as a middle-man - instead of reinventing the wheel all over again.

Nevertheless, good luck with your idea
/Estel