Thunderbird/Feature Brainstorming

Since there is now a "Feature Brainstorming" page for firefox, I'd like to start one for Thunderbird as well.

Virtual folders only.

The most important feature that I would like to see in Thunderbird is an extension of the "virtual mail folders" concept.

I would like to have all my mail physically stored exactly once.

All folders I use should be virtual in the sense that I can move any mail into any number of folders without the physical mail getting copied.

When I delete a mail, I would like to have the option to delete it from that place only (delete the link) or to delete it completely, i.e. from all folders that reference it (delete the real mail from its physical storage and thus all links to it).

The technical details only matter for performance reasons, but from a user's standpoint I do not care whether that one place is a mail folder file like the ones Thunderbird has now or a real operating system folder with one file per mail or something else, as long as I can still access my mail with operating system tools such as grep. I guess the whole concept of virtual folders could be implemented by putting all original mails into individual files in a directory, create another directory for every virtual folder and then use (hard or symbolic) links from the directory of all original mails into the virtual folder directories.

Auto filing.

When a new mail arrives that belongs to a thread which already resides in exactly one of my virtual folders, I would like the new mail to go into that virtual folder automatically.