For years the undisputed "best of breed" technologies for enterprise email encryption were Secure MIME (S/MIME) or PGP. Support for S/MIME has been built into popular email clients, going back nearly a decade. Originally considered the tool of the hacker elite, PGP has evolved into highly effective enterprise tool, usable by mere mortals. Secure systems based on both technologies are readily available, but both have a drawback that is difficult to avoid: In order to send encrypted email, the sender must be able to retrieve the recipient's public key.

Public keys are typically stored in directories accessible only to online users. Caching schemes were invented to support offline mobile users, but there's generally no way to escape the fundamental requirement; if you want to encrypt email bound for someone you've never sent email to before, you must be online to query a certificate repository or PGP key server.

By mhamrick at Cryptonomicon.

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