VM provides a special form of undo which allows changes to message attributes to be undone.
Typing C-x u or C-_ (vm-undo) undoes the last
attribute change. Consecutive vm-undo's undo further and further
back. Any intervening command breaks the undo chain, after which the
undos themselves become undoable by subsequent invocations of
vm-undo.
Note that expunges, saves and message edits are not undoable.