Not sure if it’s related, just tried to open up the source of an eml and it always opens outlook not mail.app. Even though mail.app is the preferred mail client.
Also on one machine i can install a mail plugin but on another machine it doesnt show as an option to install ??
On my system, the 10.6.7 update has removed the Devonthink bundle from Mail as incompatible and asks that I check for an updated version from Devonthink. Well, I am waiting.
Try: with Mail and DEVONthink closed, navigate to ~/Library/Mail/Bundles and delete the DEVONMailConduit bundle. Launch DEVONthink and the reinstall option should be available.