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

10249 - "The remote SMTP server answers to the EXPN and/or VRFY commands"
http://www.desknow.com/kb/idx/14/044/article/

DeskNow does not support the EXPN or VRFY commands. This is a bug in the plugin.

See this article: http://www.desknow.com/kb/idx/13/043/article/