This error can hapen if you installed DeskNow on an existing application server, which uses an older version of the Java Mail API (mail.jar).

DeskNow requires the latest version (which is still a couple of years old anyway!), but your application server may use another copy, that takes the precedence over the one installed with DeskNow under webapps/desknow/WEB-INF/lib .

For instance, this problem can happen with Tomcat, which by default uses the old mail.jar stored in /common/lib .

To solve the problem, simply replace the mail.jar used by your application server with the one that comes with DeskNow.