Not maintained anymore. Use at your own risk.

MaxColumnsPlugin 0.5

A OS X Mail.app plugin to trim trailing whitespaces and wrap your emails to 77 columns. Only tidy up plain text messages (either use "Format > Convert to plain text format" when writing your mail, or make it the default in Mail's preferences).

Please note that the default is 77 columns because this way Mail doesn't add characters to the message sent, as the Content-Transfer-Encoding header is set to 7bit.
This only works if your message uses the us-ascii charset (ie. no accents, nor quirky characters). Which doesn't mean you can't use accents, but Mail will use quoted-printable or base64 for Content-Transfer-Encoding.

Table of Content

Installation
Compatibility
I want to wrap at more / less than 77 columns
As is - No warranty
License

Installation

Note: You need python installed on your system for wrapping to work (Python comes pre-installed on Mac OS X, so it shouldn't be a problem).

Stop Mail.

Tell Mail to enable the support for bundles, if you didn't do it already:

% defaults write com.apple.mail EnableBundles -bool true

Copy the plugin MaxColumnsPlugin.mailbundle to the Mail.app plugins folder:

% mkdir -p ${HOME}/Library/Mail/Bundles/
% cp -r MaxColumnsPlugin.mailbundle ${HOME}/Library/Mail/Bundles/

Restart Mail.

Compatibility

OS X Lion 10.7.4
Mail.app 5.3 + Message.framework 5.3
OS X Mountain Lion 10.8
Mail.app 6.0 + Message.framework 6.0

You can enable the plugin to support other versions of Mail with the following procedure (but keep in mind that it will not have been tested):

Open up Terminal (/Applications/Utilities/Terminal.app) and type

defaults read /Applications/Mail.app/Contents/Info PluginCompatibilityUUID
defaults read /System/Library/Frameworks/Message.framework/Resources/Info PluginCompatibilityUUID

This should print to UUIDs like these ones:

4C286C70-7F18-4839-B903-6F2D58FA4C71
EF59EC5E-EFCD-4EA7-B617-6C5708397D24

Add these to the SupportedPluginCompatibilityUUIDs value in the bundle's Info.plist file (MaxColumnsPlugin.mailbundle/Contents/Info.plist). This is what the Info.plist looks like with the Mail and Message.framework 5.3 UUIDs:

<key>SupportedPluginCompatibilityUUIDs</key>
<array>
    <string>EF59EC5E-EFCD-4EA7-B617-6C5708397D24</string>
    <string>4C286C70-7F18-4839-B903-6F2D58FA4C71</string>
</array>

I want to wrap at more / less than 77 columns

Just edit the MCMaxColumnsWrap value in the bundle's Info.plist file (MaxColumnsPlugin.mailbundle/Contents/Info.plist).

<key>MCMaxColumnsWrap</key>
<integer>77</integer>  # Put the value you want here

As is - No warranty

TL;DR : If this breaks your system / your mails, don't blame me.

The program is distributed in the hope that it will be useful, but without any warranty. It is provided "as is" without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose. The entire risk as to the quality and performance of the program is with you. Should the program prove defective, you assume the cost of all necessary servicing, repair or correction.