This bundle is currently not part of the official MailMate package, so you can install it with:
mkdir -p ~/Library/Application\ Support/MailMate/Bundles
cd ~/Library/Application\ Support/MailMate/Bundles
git clone https://github.com/ats/atom.mmbundle
Then restart MailMate.
If not otherwise specified (see below), files in this repository fall under the following license:
Permission to copy, use, modify, sell and distribute this
software is granted. This software is provided "as is" without
express or implied warranty, and with no claim as to its
suitability for any purpose.
An exception is made for files in readable text which contain their own license information, or files where an accompanying file exists (in the same directory) with a "-license" suffix added to the base-name name of the original file, and an extension of txt, html, or similar.