mu4e includes a number of example actions in the file mu4e-actions.el
in the source distribution (see C-h f mu4e-action-TAB). For example, for
viewing messages in an external web browser or tagging.
It is easy to add such actions to your configuration; for instance, to enable tagging19, you could add:
(add-to-list 'mu4e-headers-actions
'("Tag message" . mu4e-action-retag-message))
(add-to-list 'mu4e-view-actions
'("Tag message" . mu4e-action-retag-message))
mu4e does not offer tagging by default since it
mutates the message files, something that mu/mu4e generally try to
avoid. An alternative to tagging is labeling, See Applying and clearing labels