The crux of this hint (finding the system idle time) was not my doing. I don't claim credit for it, but I forget where I found it. I think it was on an AppleScript bulletin board or mailing list. I'm sure there are multiple uses for the basis of this script, but I wanted it for the reason stated above. Copy and paste the script into Script Editor. Then save the script, and you'll then need to either set an iCal event or a cron task to run the script at one or two times a day. Here's my cron entry:
0 8,23 * * * /path/to/script.scpt
This runs the script at 8am, which will launch Mail, and again at 11pm, when the idle time is monitored and Mail is quit if I'm not using the computer. This is a better solution for me than the screensaver option I have seen. That solution only activates at the time I specify and not every time I let the computer go to sleep.

