When the mailbox is sorted by threads, there are a few additional functions available in the index and pager modes.
D delete-thread delete all messages in the current thread
^U undelete-thread undelete all messages in the current thread
^N next-thread jump to the start of the next thread
^P previous-thread jump to the start of the previous thread
^R read-thread mark the current thread as read
ESC d delete-subthread delete all messages in the current subthread
ESC u undelete-subthread undelete all messages in the current subthread
ESC n next-subthread jump to the start of the next subthread
ESC p previous-subthread jump to the start of the previous subthread
ESC r read-subthread mark the current subthread as read
ESC t tag-thread toggle the tag on the current thread
See also: $strict_threads.