I was doing some googling looking for a Gmail applet for Gnome, and found the project page of the (probably dead) application “Gmail” (short for “Gnome Mail”):
Gmail is an experiment in a sql vfolder based email system. It implements sql vfolders, incoming POP3, outgoing SMTP and more.
Hmm… SQL based VFolders. Sounds a lot like “labels” in Google’s Gmail to me…