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))