I am looking for a small footprint (in terms of processor cycles) Mail Transfer Agent for a Linux box. The box is actually a VPS server hosted at Linode. I am building the server to replace all the services that are currently delivered on a shared hosting account. I am running a basic LAMP server. One of my web applications sends emails to users and I have any return emails piped to a php script. All I'm looking for is something lightweight that I can easily configure to pipe these emails to a php script. If it can also act as an outgoing smtp server that would be great too.
I currently have the server running ubuntu 8.04 LTS, but I am not that far down the road, and have many choices if there is a better one for this situation.