mbox maildir/mh pop imap notmuch nntp gmail mbsync/offlineimap
add label set label remove label complete (add) complete (remove) toggle label complete (toggle) quasi-(label/hide)
local mbox remove mbox motmuch mbox (limit to lowest common denominator?)
header (file) DB (notmuch) DB (imap)
flags preserved flags changed type flags munged flags lost flags duped?
header flags server (db) flags header flags (diff from parent) server flags (diff from parent)
tag/tagged flag/flagged label keyword
promote legacy header fields? rename notmuch tags? sorting when both types present?