Email Encryption -Server Side

Hi,

I need to encrypt our office emails.We use Outlook 2007, and mailenable mail server.

Is there a way i can do it from the server side only(mail server) without necessarily having to configure outlook in every client.

Is so, or if not what do i need.. certificates? where do i get for a digital certificate for free and how do i go about in creating the digital signatures

Help need.

Regards