If you've just upgraded to OS X 10.8.5 and you use the TruePreview plug-in for Mail.app, you will notice that TruePreview has been disabled. That's because Mail.app has now been updated from v6.5 to v6.6. The last time this happened, I created a write-up for a workaround that gets TruePreview up and running. Hopefully, it will save you some trouble if you're in the same situation. I take no credit for this at all - it's a combination from several different posts I found scattered around the lower entrails of Google 
GETTING TRUEPREVIEW WORKING IN MAIL.APP:
Whenever Mail.app is updated, the update always breaks TruePreview until a new version of TruePreview is eventually released, or you can just follow this procedure rather than waiting:
1. The first thing you need to do is find out what the UUIDs of the new Mail.app version are. To do so, enter the following two commands, one after the other, in Terminal:
3. Make sure you quit Mail.app. You'll find the TruePreview.mailbundle in ~/Library/Mail/Bundles (Disabled). Move that bundle to ~/Library/Mail/Bundles
4. You need to edit the ~/Library/Mail/Bundles/TruePreview.mailbundle/Contents/info.plist file.
5. Open the info.plist file in a text editor (TextEdit, TextWrangler, etc), and find the string SupportedPluginCompatibilityUUIDs.
6. Below that, you should see a list of strings that look like the UUIDs you found for Mail.app and Message.framework in the Terminal window.
7. Following the same format as those other strings, add the UUIDs for the new version of Mail.app (don't forget the <string> and </string> tags).
8. Save the file, restart Mail, and TruePreview should be back in business.
GETTING TRUEPREVIEW WORKING IN MAIL.APP:
Whenever Mail.app is updated, the update always breaks TruePreview until a new version of TruePreview is eventually released, or you can just follow this procedure rather than waiting:
1. The first thing you need to do is find out what the UUIDs of the new Mail.app version are. To do so, enter the following two commands, one after the other, in Terminal:
defaults read /Applications/Mail.app/Contents/Info PluginCompatibilityUUID
defaults read /System/Library/Frameworks/Message.framework/Resources/Info PluginCompatibilityUUID
2. That should return two strings that are the new UUIDs for Mail.app. You then need to add those strings to the info.plist file found in TruePreview.mailbundle.3. Make sure you quit Mail.app. You'll find the TruePreview.mailbundle in ~/Library/Mail/Bundles (Disabled). Move that bundle to ~/Library/Mail/Bundles
4. You need to edit the ~/Library/Mail/Bundles/TruePreview.mailbundle/Contents/info.plist file.
5. Open the info.plist file in a text editor (TextEdit, TextWrangler, etc), and find the string SupportedPluginCompatibilityUUIDs.
6. Below that, you should see a list of strings that look like the UUIDs you found for Mail.app and Message.framework in the Terminal window.
7. Following the same format as those other strings, add the UUIDs for the new version of Mail.app (don't forget the <string> and </string> tags).
8. Save the file, restart Mail, and TruePreview should be back in business.