mu4e supports a number of marks:
mark for/as | keybinding | description
-------------+-------------+------------------------------
delete | D, <delete> | delete
move | m | move to some maildir
refile | r | mark for refiling
-------------+-------------+------------------------------
flag | + | mark as 'flagged' ('starred')
unflag | - | remove 'flagged' mark
read | ! | mark as read
unread | ? | marks as unread
trash | d | move to the trash folder
untrash | = | remove 'trash' flag
-------------+-------------+------------------------------
label | l | change the labels
unlabel | L | clear all labels
action | a | apply some action
'something' | *, <insert> | mark now, decide later
-------------+-------------------------------------------
unmark | u | remove mark at point
unmark all | U | remove all marks
-------------+-------------------------------------------
|
mu4e-refile-folder and especially See Smart refiling for details.
mu4e-headers-actions