Digital certificate is an electronic document hat establishes your credentials when doing business or other transactions on the Web. It is issued by a certification authority, and can expire or be revoked. It contains your name, a serial number, expiration dates, a copy of the certificate holder’s public key (used for encrypting messages and digital signatures), and the digital signature of the certificate-issuing authority so that a recipient can verify that the certificate is real. It also ensures that contents have not been altered in transit. A user can prevent tampering of e-mail messages in Microsoft Office Outlook 2007 by using S/MIME. Secure Multipurpose Internet Mail Extensions is a specification for secure e-mail messages that uses the X.509 format for digital certificates.