U.S. Patent No.7,406,501
  System and method for instant messaging using an e-mail
  protocol

  Abstract
  Systems and methods allowing an instant messaging user to exchange messages with an e-mail user. 
  To the instant messaging user, the experience is a seamless exchange of instant messages; to the 
  e-mail user, the experience is a seamless exchange of e-mail messages. Conversion of an instant 
  message to an e-mail message includes insertion of a token into the e-mail message, and conversion
  of an e-mail message to an instant message includes validating a token extracted from the e-mail 
  message.