This is typically (so I've seen on my server) caused by IIS recycling the Application Pool process. You can specify an exact time (or set of times) to force a recycle, otherwise, IIS should hold the process open.
Open IIS Manager, find the Application Pool SmarterMail, and open Advanced Settings.
Under Recycling, find Specific Times, and put in at least one time when the AppPool can be recycled. I use 4:00a because it's a very low-use time of day.
Since I added that setting, I only see that "error"/notice once a day at 4:00a.