Use Notmuch for mail everywhere in Emacs, including
compose-mail (‘C-x m’)
compose-mail-other-window (‘C-x 4 m’)
report-emacs-bug
org-submit-bug-report
and other entry points.
(setopt read-mail-command #'notmuch
mail-user-agent (progn (require 'notmuch)
'notmuch-user-agent))