How to make emacs style ASCII-art table in Mail?
As suggested by patrix you could do that in Emacs. To automate it, you should use QuickCursor (available in the Mac App Store), and a special build of Emacs, which you can do by checking out the latest source code from ftp://alpha.gnu.org/gnu/emacs/pretest and then using the patches available from ftp://ftp.math.s.chiba-u.ac.jp/emacs

Instruction on how to build are available in the archive containing the patch file. Once you've done that, editing between Emacs and Mail (or whatever else), should be extremely easy.