The ideal system that everyone is searching for - the silver bullet, is to have top security automatically regardless of who you are sending to and what product(s) they happen to be using. The reality is that many e-mail packages are not themselves secure, and do not interoperate cleanly with anything but their own products. For the time being you are better off keeping your security outside of your e-mail or word processing package, and exchanging attachments that are fully protected and not relying upon any of the different systems that people are using. That way you increase the security of the result and do not have to rely on complex interactions between proprietary systems.
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