This guide will cover the basics on how to integrate emacs with the bridge using

• mu4e
• mbsync
• smtpmail

How to install them:

• Fedora: sudo dnf install isync maildir-utils
• Debian: sudo apt install isync maildir-utils

Create a configuration file ~/.mbsyncrc for mbsync

IMAPAccount protonmail
Host 127.0.0.1
Port 1143
PassCmd "gpg -q --for-your-eyes-only --no-tty -d ~/.authinfo.gpg | awk 'FNR == 1 {print $8}'"
SSLType STARTTLS
SSLVersions TLSv1.2
CertificateFile ~/.config/protonmail/bridge/cert.pem

IMAPStore remote
Account protonmail

#You can change .mail to something else
MaildirStore local
Path ~/.mail/
Inbox ~/.mail/INBOX/

Channel inbox
Master :remote:
Slave :local:
Patterns * 
Create Both
Expunge Both
SyncState *

Group protonmail
Channel inbox

Next we need to create a file to log our credentials for sending mail with smtpmail. Create a file ~/.authinfo with the following contents

machine 127.0.0.1 login USERNAME_HERE@protonmail.com port 1143 password PASSWORD_PROVIDED_BY_BRIDGE
machine 127.0.0.1 login USERNAME_HERE@protonmail.com port 1025 password PASSWORD_PROVIDED_BY_BRIDGE

You should encrypt it one option is to use epa in emacs M-x epa-encrypt-file. Another is to set a symmetric encryption that will require some password $ gpg --symetric .authinfo. Finally delete .authinfo.

Now to get/sync you mail, run on a terminal $ mbsync protonmail.

This is a minimal configuration that you should include in your init.el configuration file.

(require 'mu4e)

(setq mu4e-maildir "~/.mail"
mu4e-attachment-dir "~/Downloads")

(setq user-mail-address "USERNAME_HERE@protonmail.com"
user-full-name  "YOUR_NAME")

;; Get mail
(setq mu4e-get-mail-command "mbsync protonmail"
mu4e-change-filenames-when-moving t   ; needed for mbsync
mu4e-update-interval 120)             ; update every 2 minutes

;; Send mail
(setq message-send-mail-function 'smtpmail-send-it
smtpmail-auth-credentials "~/.authinfo.gpg"
smtpmail-smtp-server "127.0.0.1"
smtpmail-stream-type 'starttls
smtpmail-smtp-service 1025)

(add-to-list 'gnutls-trustfiles (expand-file-name "~/.config/protonmail/bridge/cert.pem"))

To index your emails from the terminal run mu index --maildir=~/.mail

Now just launch M-x mu4e from emacs to read your mail.