When Apple released the most recent 10.6.8 security patch, that patch broke Letterbox (see also here), an insanely useful Mail.app plugin that allows all three Mail.app panels to be viewed vertically. This view maximizes screen real estate, which is very important for those of us on widescreen displays—which is to say, virtually all Mac users. But this 2010 OS X Daily post describes how to work around the last breakage caused by an Apple update. These are their instructions, except for the addition of two new UUIDs that I found for the latest version of 10.6.8:

* From the Finder, hit Command+Shift+G and enter ~/Library/Mail/ then hit Go
* Open Bundles (Disabled) rather than Bundles – note: if you have already opened Mail, the plugin is disabled, if you haven’t opened Mail yet, it will be in Bundles
* Right-click on Letterbox.mailbundle and select “Show Package Contents”
* Now open the “Contents” folder inside the Letterbox.mailbundle contents
* Using a text editor, open Info.plist (you can use TextEdit, don’t use Word)
* Scroll to the bottom of the Info.plist file and look for “SupportedPluginCompatibilityUUIDs” which is surrounded by key tags, below that will be a bunch of hex strings surrounded by string tags
* Add the following two strings to the bottom of the list (inside the array tags):

<string>064442B6-53C0-4A97-B71B-2F111AE4195B<string>
<string>588FF7D1-4310-4175-9980-145B7E975C02<string>

That’s the important part. The rest is fairly simple:

* Save these changes to the Info.plist file
* Go back to the Mac OS X desktop and hit Command+Shift+G again, then enter ~/Library/Mail/
* You’ll see these two folders again: Bundles and Bundles (Disabled), what you need to do is move the Letterbox.mailbundle plugin from the (Disabled) folder to the Bundles folder. Do this just by dragging the file from one folder window to the other.
* Relaunch Mail.app

You can also navigate to the folder ~/Library/Mail/Bundles on your own, without using the “Go” command.

A lot of people—especially the nerds likely to use Letterbox—have probably already moved to 10.7 or 10.8, though I still haven’t and am unlikely to in the foreseeable future.