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Securing e-mail communication across clients/OSs

Dear lazyweb: I am in a dreadful situation! I need to secure e-mail communication between five types of users:

To make matters worse: there is no common provider or server infrastructure, so the solution must work across providers and mailboxes.

Requirements

Ideally, the solution will be open-source. However, if proprietary software performs better, then we will gladly use that where required.

Previous attempts

So far, we've tried (and were let down by)

Help!

Does anyone have proper suggestions over what to use?

In fact, if you are interested in devising a solution, or even deploying it, there's money to pay you for those services. Write in if you are interested.