This is with Mail Version 12.0 (3445.100.17) on Mac Mojave 10.14 Beta (18A336e). Mail is not picking up any mail-plugin (foo.mailbundle) placed at ~/Library/Mail/Bundles. So the mail plugin is not activating.
Anybody else seeing a similar issue?
Workaround/Fix I found:
Running the following commands fixed it:
cd ~/Library/Containers/com.apple.mail/Data/DataVaults
# Set symbolic link -- get "MailBundles" to point to root
ln -s / MailBundles
# re-start Mail
Thanks
-Vivek