OK, let me just state at the outset that I am not a developer, so I have no access to Mavericks 10.9. My guess would be that there is a new version of Mail.app in Mavericks, so the UUID's will not be same as posted by Bammi, since those are the Mail.app UUID's for Mountain Lion 10.8.4.
To find out if there are new Mail.app UUID's in Mavericks, you can try running these 2 commands:
defaults read /Applications/Mail.app/Contents/Info PluginCompatibilityUUID
defaults read /System/Library/Frameworks/Message.framework/Resources/Info PluginCompatibilityUUID
Hopefully that should return two different strings that are the new UUIDs for Mail.app running under Mavericks. You then need to add those strings to the info.plist file found in TruePreview.mailbundle, following the same instructions posted above by Bammi. Hope that helps, but like I said, I have no access to Mavericks, so I cannot confirm whether in fact the Mail.app UUID's have been updated.