Thunderbird:Replace Normal Folders with Virtual Folders

When using Virtual Folders, Thunderbird will save all messages into a single database. Each message will have a set of metadata associated with it, such as From, To, Account, Junk Status etc. The folder hierachy will be constructed from these metadata.

A new metadata field, keywords, will be provided for the user to add customized labels/tags to each message. They can add keywords by typing them in, through filters or by dragging them to a folder. Each normal folder will be associated with 1 or more keywords, but not other metadata (however, special folders, such as the junk folder, can associated with any metadata, including keywords). This basically means a particular message can appear under several folders.

I believe this system is far more flexible than the current system and it works better with database, which will be used as the storage backend for Mozilla 2.0

I'm not sure whether it's ever been clarified that we'll be switching from mbox-based mail storage to SQLite-based storage. If indeed we are, it might be worth considering adding a mail export feature at the same time so that prospective users will feel more comfortable testing out Thunderbird without having to lose some of their mail (because it's in a mail system which isn't importable existing mail clients.

Along the same lines, it's worth considering how mail will be exported when it doesn't truly have one parent folder. If there are multiple parent folders (categories, labels, what-have-you), which one will store the mail when it gets exported? (This assumes export happens at the same time as this feature's added, per above -- if it doesn't, it's still a consideration for when export is added.)--Waldo