I’ve been using Microsoft Outlook 2011 on Mac OS X for a couple years and I finally decided I needed to give Mac Mail and iCal a try.  In Lion (Mac OS X 10.7), Mac Mail has added some great features, but one thing it is missing is including the headers of the original message when replying.  Here is an example of what Mac Mail shows:

On Jun 14, 2012, at 1:03 PM, Geoff Walker wrote:

Outlook shows this:

From: John Smith [mailto:jsmith@example.com] Sent: Thursday, June 14, 2012 12:41 AM
To: MyTeam
Subject: Yesterday’s meeting

I found an add-on that will modify Mac Mail to do the same thing.  Saptarshi Guha wrote ReplyWithHeaders, which is a bundle that can be added to Mac Mail to modify its behavior, and released the source on GitHub.  Here are the steps to use it if you have access to Xcode (the development tool for Mac OS X and iOS):

1. Clone the ReplyWithHeaders repository by using the following command:

   git clone https://github.com/saptarshiguha/ReplyWithHeaders.git ReplyWithHeaders

2. Quit out of the Mail app.
3. Build the ReplyWithHeaders project in Xcode.  This will place the ReplyWithHeaders.mailbundle file in the right place, which is in ~/Library/Mail/Bundles directory.
4. From a Terminal window, execute the following commands:

   defaults write com.apple.mail EnableBundles -bool true
   defaults write com.apple.mail BundleCompatibilityVersion 4

5. Start the Console app to help you verify that the installation worked.  In the search bar in the upper right corner of the Console app type ‘Mail’.
6. Restart the Mail app.
7. In the Console app you should see the following lines:

   ReplyWithHeaders: Oh its a wonderful life
   Loaded ReplyWithHeaders

You can also find these instructions on the webpage for the bundle.  Pretty cool, huh?