As tilly points out, what needs to be done will depend on which OS you have. Apart from that, once you have the appropriate cron/scheduler solution that would make sure your program is always doing its job, it would probably be better to restructure your program as a "one-shot" thing instead of an "infinite-loop-sleep-for-5-minutes" thing.

That is, just set up a schedule that will run this monitoring script once every 5 minutes, and emails will be sent at that interval when appropriate. (If it needs to preserve state information from one interval to the next, just store that in a disk file.)