On one of our production servers, we have found that the SmptProxy service will sometimes crash and the service will stop.
 
We are researching the problem now and will post a fix when we have tested it.
 
As a work-around, do the following:
1) Go to the list of Windows services. It is usually under Control Panel -> Administrative Services -> Services
2) Right click on the SmtpProxy service and choose Properties
3) Choose the Recovery tab
4) Change first, second, and subsequent failures to "Restart the service"
5) Change restart after to 0 minutes
6) Click Ok to close the window and save your changes
 
Now, if the service does fail, it should restart automatically.