Catch emails to all a_user_name@mydomain.com, and execute code

In my webapp I have instances where I want to let users send an email to their local representative. But I don't want to expose that user's email so I'd like to have the email sent from a_user_name@buycott.com, where a_user_name is different depending on who sends it.

Where I'm lost is how to handle the situation where somebody replies to that email, and I need to deliver it to the original author. I'd like to be able to catch that email, look up the user in our database, and send them an email notifying them of the reply. How do I complete this last part?

Using Rails + Amazon SES

Thanks!