Digital IDs and Signatures

Having a digital ID enables you to add another layer of security to your e-mail via digital signatures. A digital ID ties your identity information—name or e-mail ID, for instance—with your public key.
When you use this ID to digitally sign your messages, a part of your message is encrypted with your private key, so that the recipient knows that the message came from you; if you encrypt the message in addition, then the signature and the message are encrypted with your public key. This enables the recipient to know whether the message has been accessed or modified en route.
Digital IDs are provided via e-mail certificates, which are usually issued by external certification authorities (CAs). In organizations, sometimes, the administrator of your Exchange Server generates these certificates for users.
If you use Outlook, you can get e-mail certificates from CAs. Some like Comodo offer these free of charge for personal use; for commercial use, different schemes are available from various CAs. source:India syndicate