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

java.lang.OutOfMemoryError: unable to create new native thread
http://www.desknow.com/kb/idx/6/181/article/

This message means that you have probably set DeskNow to use too many threads.

Check the number of simultaneous SMTP Incoming connections, POP3 connections, delivery threads, IMAP connections.