Found this to be useful.
52D2D439-8589-4291-A03A-79F51A9234D9
On a related note, my script to find the latest UUIDs seemed to not work with this pre-release. Anyone know why?
This is part of my script:
defaults find UUID | grep MailCompatibility
defaults find UUID | grep MessageCompatibility
Found using "older" method:
defaults read /Applications/Mail.app/Contents/Info PluginCompatibilityUUID
Cheers!