This is a commercial product, probably one of the most robust ones out there

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. It originally started as a free computer security solution, offering a very advanced encryption standard to everyone for file encryption. Once it expanded, it covered a variety of computer security applications, including email encryption. The program is quite complicated and not so much for the novice user, but it is an expert's delight. PGP uses state-of-the-art encryption techniques, including assymetric ones for the emails. It offers a lot of options and the GUI it has is very good. Probably a worthwhile solution for a medium to large company, due to its high cost. PGP (along with its other commercial variations) is now part of Symantec.