Re: using a cron job...

Most mail-delivery-agents allow you to set up a "magic" mailbox to run a script as well. E.G. you might have johndoe-blessing@church.org directed to execute your script on delivery. Examples include QMail and Procmail.

This is just a more general case that would allow "any time" changes. We used to use a QMail-triggered script to check replies to certain automatic eMails, something like the forms you send to hostmaster@internic.net