I have written a Python script that checks a certain e-mail address and passes new e-mails to an external program.  How can I get this script to execute 24/7, such as turning it into daemon or service in Linux.  Would I also need a loop that never ends in the program, or can it be done by just having the code re executed multiple times?
cron is clearly a great choice for many purposes.  However it doesn't create a service or daemon as you requested in the OP.  cron just runs jobs periodically (meaning the job starts and stops), and no more often than once / minute.  There are issues with cron -- for example, if a prior instance of your script is still running the next time the cron schedule comes around and launches a new instance, is that OK?  cron doesn't handle dependencies; it just tries to start a job when the schedule says to.
If you find a situation where you truly need a daemon (a process that never stops running), take a look at supervisord.  It provides a simple way to wrapper a normal, non-daemonized script or program and make it operate like a daemon.  This is a much better way than creating a native Python daemon.
How to make a Python script run like a service or daemon in Linux