Gary, my personal recommendation on this is to leverage an SMTP based sensor (such as the one found in PRTG) to periodically connect, authenticate, and perhaps even send a test message. If any part of this process fails you can typically assume the SmarterMail service has stopped or the server is otherwise unavailable.
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com