From: DeskNow Knowledge Base
http://www.desknow.com/kb/

Does DeskNow support SMTP VRFY or EXPN commands?
http://www.desknow.com/kb/idx/13/043/article/

DeskNow does not support these commands, for security reasons.

You can test this directly by using telnet on port 25 on your server, and typing commands like:

VRFY joe

or

EXPN people

Note: some Nessus plugins wrongly report that DeskNow supports these commands. This is a Nessus problem.