Now that you have a certificate on your system, you’re ready to start digitally signing your outgoing messages so that recipients can verify your identity. When you send a digitally signed message, Outlook 2007 sends the original message and an encrypted copy of the message with your digital signature. The recipient’s e-mail application compares the two versions of the message to determine whether they are the same. If they are, no one has tampered with the message. The digital signature also enables the recipient to verify that the message is from you.
Because signing your e-mail requires Outlook 2007 to send two copies of the message (the unencrypted message and the encrypted copy), the signed e-mail message is larger.