Secure mail consists of two parts, digital signatures and encryption.
A digital signature is a method used to ensure that a message was not altered in transit and verifies the signer to be whom they claim to be. The recipient for whom the message was encrypted for is the only one that can decrypt the data. A message can be singed only or encrypted only, but most commonly, a message that is important enough to secure with one is important enough to secure using both. Copyright Worldtalk Corporation 1999.