Having selected one or more emails with labels and clicking on the "label" button, to add or remove labels to the emails selected in the set, the list of labels in the label selection box should be grouped and displayed as follow:
- first the labels attached to the whole emails in the set;
- then the labels attached to just some of the emails in the set;
- then the labels not attached to mails in the set.
The three groups of labels shall respect the user label sorting.

Doing so it helps a lot with copying or removing labels among similar mails, selected in the set, before clicking of the "label" button.