Best self-hosted autoresponder? (php only)

I need to host autoresponders for numerous small businesses. I know about Infinite Autoresponder, but it's not the most usable system. For one thing, it doesn't allow the user to change the order of the responses, or insert new ones. The user interface is also confusing.

So I'm looking for an self hosted autoresponder for my server. It has to be written in PHP, because I'm going to adapt it to suit my needs. Usability is probably the most important feature. I don't care if it's paid or free - I probably prefer paid for support purposes.

Any suggestions?

Thanks.

Lemonade


P.S. If you're more technically minded, here's some additional information:

I'm going to adapt it to use one install for multiple sites. It would help if it had the following features:
- No IonCube/Zend obfusicated code. I need to be able to change the code.
- Multiple admins (I'll hack it so there's a super user admin, and a website admin).
- Handles multiple sites from one install.
- Uses SMTP servers instead of built-in PHP mailing.
- Master resale rights would be cool - what I have in mind, a lot of people might be interested in.