Sounds interesting. A bit like the tagging system in Thunderbird? Guessing it won't sync over IMAP though which would be a shame...
Just seen this list of features:
http://doc.runbox.com/twiki/bin/view...ComingFeatures
Any news on Full message body searching? I'd be very interested in being able to search sub-folders (ie search Archive and it gets all items from Archive/2009, Archive/2008, and even Archive/2010/January etc), even if it was just subject and from/to etc