Use SSL. Using SSL will encrypt the data for your email. You can also you keys that sign/encrypt messages but you have to distribute your public key and obtain the public keys of the people you want to communicate with.

As I wrote in a related question earlier, use S/MIME, it provides authentication, message integrity and non-repudiation of origin (using digital signatures) and privacy and data security (using encryption). Before S/MIME can be used, you must obtain and install an individual key/certificate either from an in-house certificate authority (CA) or from a public CA.

And, as for open source email platforms - I guess you mean email clients you can use instead of Outlook, there are a few, here are three that come highly recommended:

Claws Mail - Highly recommended for netbook user due to lightweight usage.

Mozilla Thunderbird - The wide variety of add-on for this email client makes it very flexible, secure and easy to use. Highly recommended for desktop and power users.

Zimbra Collaboration Suite ~ Open Source Edition or Businesses Editon - Highly recommended for business and corporate users due to its support for a broad range of email clients and mobile devices via "over the air" sync.

If you meant email servers that can replace Exchange, I heard hmailserver was pretty good, otherwise you will have to turn to Linux for a reliable email server.

You have to define what's best for you. PGP and S/MIME are usually mentioned, but they are typically too difficult for non-technical people.

One good option is to find web-based solution, which integrates into email-client and the receiver will get the SSL-protected link to the message. Easy to use and usually secure enough.

If you don't need encryption from user-to-user, but network-to-network is enough, I would consider configuring mail-servers to use SSL/TLS.

SecureZIP from PKWARE. End to End Compression and Encrytion. www.PKWARE.com