mdadm, but it took me a couple of separate attempts to get it configured to use Gmail as its SMTP relay. There are a lot of good Postfix tutorials out there, but this post will detail the steps I took for my particular situation. I'm currently running Ubuntu 11.10 Server 64-bit, so I needed mdadm to be able to send me notification emails, and I already had CA and server certs for Apache and ProFTP, so I'll skip those steps here.