Gmail integration would be great because I use Gmail and wouldn't have to use some other app.

So if this were a rails app, there'd be two models. You'd have Message Templates and Senders.

Message Templates would be subject line, body, signature, and any file attachments.

A sender would consist of an e-mail address, a selected message template, and then some sending settings (how long to wait before writing back, for example). There'd be a button for a sender to be "active" or inactive. If a sender is "active" and has a message template associated with it, then when an e-mail comes in, the sender sends back the template tied to it.

Then maybe each sender would have some stats about how its performing, how many times it executed, what the open rate on those e-mails was, if there was a response etc etc.