Microsoft Exam 98-367 Topic 1 Question 53 Discussion

By digitally signing a message, you apply your unique digital mark to the message. The digital signature includes your certificate and public key. This information proves to the recipient that you signed the contents of the message and not an imposter, and that the contents have not been altered in transit.

http://office.microsoft.com/en-us/outlook-help/secure-messages-with-a-digital-signature-HP001230539.aspx