Why not send up an on screen message about high uptime rather than an email that can be ignored or missed? If the Macs are already in a Smart Group, you can have a policy just send up a notification, like with jamfHelper in a script, or with the jamf binary, or some 3rd party tool, etc.
It's not that easy to send emails to clients from the console without doing things like having the Mac send an email to the user of that system in a policy, which involves figuring out the client's email address, from the local account or some other method.
One word of caution on this. Since uptime is something that can change very quickly, and I presume you use an EA to capture the uptime, which only gets updated on inventory collection, I would recommend using a script to first check uptime, live, to be sure it's in the high range you expect it to be, before sending up any kind of message on screen. This would avoid nagging someone about rebooting their Mac, shortly after they just rebooted, which isn't a great user experience.
