This will help you setup DKIM: http://www.dnswatch.info/dkim/create-dns-r...
mdadm is an easy-to-use software RAID utility for Linux.
Vim is a text editor that is very simple and lightweight, but extremely powerful. This article provides some tips and how-to's to get started.
Bash is the "Born Again SHell" (BASH) written by Brian Fox and is the default shell on many unix systems.
Unix Users and Groups
A quick reference for how to add and manage UNIX users and groups.