A complete and fully-working (including STARTTLS encryption and HTML emails) setup to use Proton Mail Bridge with emacs

gnus-proton-mail-layer.el

(defun user/mail-layer ()
    "Set GNUS up with asynchronous mail sending, and Proton Mail support,
    and a relatively modern layout!
    
Loads:
- `async': for asynchronous mail"
    (use-package async
        :demand t
        :config
        (load "smtpmail-async.el")
        (setq message-send-mail-function 'async-smtpmail-send-it)
        (setq send-mail-function 'async-smtpmail-send-it))
    
    (with-eval-after-load "gnus"
        (require 'imap)
        (require 'gnus-registry)

        (setq nnmail-expiry-wait 'immediate)

        (setq gnus-select-method '(nnimap "localhost"
                                          (nnimap-address "127.0.0.1")
                                          (nnimap-server-port 1143)
                                          (nnimap-stream starttls)))
        (setq mail-sources '((imap :server "127.0.0.1"
                                 :user "<your full email address>"
                                 :password "<password from Bridge>")))
        (setq gnus-summary-mark-as-read-forward t)
        (setq gnus-parameters '((".*" (display . all))))
        (setq gnus-inhibit-images nil)
        (setq gnus-large-newsgroup nil)

        (setq smtpmail-debug-info t)
        (setq smtpmail-debug-verb t)
        (setq smtpmail-default-smtp-server "127.0.0.1")
        (setq smtpmail-smtp-server "127.0.0.1")
        (setq smtpmail-stream-type 'starttls)
        (setq smtpmail-smtp-service 1025)
        (setq smtpmail-smtp-user "<your full email>")
        (setq user-mail-address "<your full email>")
        (setq smtpmail-servers-requiring-authorization ".*")
        (setq user-full-name "<your full email>")
        (require 'smtpmail)

;;;; Gnus Window Layout
;;;;; Layout when just viewing the summary view
        (gnus-add-configuration
         '(summary
           (horizontal 1.0
                       (vertical 25 (group 1.0))
                       (vertical 1.0
                                 (summary 1.0 point)))))
;;;;; Layout when gnus is showing an individual message
        (gnus-add-configuration
         '(article
           (horizontal 1.0
                       (vertical 25 (group 1.0))
                       (vertical 1.0
                                 (summary 0.25 point)
                                 (article 1.0)))))
;;;;; Layout when composing a message
        (gnus-add-configuration
         '(message
           (horizontal 1.0
                       (vertical 25 (group 1.0))
                       (vertical 1.0
                                 (summary 0.25 point)
                                 (message 1.0)))))
;;;;; Layout when replying to message
        (gnus-add-configuration
         '(reply
           (horizontal 1.0
                       (vertical 25 (group 1.0))
                       (vertical 1.0
                                 (summary 0.25 point)
                                 (article 0.3)
                                 (reply 1.0)))))
        (add-hook 'message-mode-hook (lambda ()
                                         (+core--internal-local-map!
                                             "S" #'message-send-and-exit
                                             "H" #'org-mime-htmlize)
                                         (general-nmap
                                             "q"   #'message-kill-buffer))))
;;;; Get email notifications on the desktop!
    (use-package gnus-desktop-notify
        :defer 10
        :config
        (gnus-desktop-notify-mode)
        (gnus-demon-init)
        ;; Check for new mail once every five minutes if Emacs has
        ;; been idle for two minutes
        (gnus-demon-add-handler 'gnus-demon-scan-mail 5 2)
        ;; Check mail once a day mid day
        (gnus-demon-add-handler 'gnus-demon-scan-mail "12:00" nil)
        ;; Make sure to time-out gnus
        (advice-add 'gnus-demon-scan-mail
                    :around (lambda (oldfun)
                                (with-timeout (120 (message "Gnus timed out"))
                                    (funcall oldfun time idle)))))
;;;; Send HTML emails via org-mode
    (use-package org-mime
        :commands (org-mime-htmlize)))