Growing pains

Setting up mu4e for office 365

A few months ago, my work email switched from outlook to office365 and with it came OAUTH2. The transition was coupled with a lack of initial support for receiving email with this security protocol yielding some email clients useless. The goal of this post is to provide future users with some insights on setting up OAUTH2 in mu4e.

What is OAUTH2?

In brief, OAUTH2 is an authorization framework that enables applications to obtain limited access to user accounts through an HTTP service; applications can request access to user accounts by setting up a host-client secret. In my mind it works similar to ssh, but then for email.

Setting up OAUTH2 in emacs

We need to achieve 3 functions; we need to acquire a secret, use this secret to receive the emails, use this secret to send the emails. As an important aside, it is highly recommended to set up some form of password management such that your passwords are not visible of left strangling in clipboard; setting this up is outside the scope of this post.

Acquiring the secret

Acquiring the secret may differ depending on the service you are using. For gmail for example look here. For office 365 however, it proved a bit more difficult as I did not have access directly to the api to setup these keys through azure. In order to acquire a secret I used, and I used the publically available application id for thunderbird which can be found here. At the time of writing these are

AppID = "08162f7c-0fd2-4200-a84a-f25a4db0b584"
ClientSecret = "TxRBilcHdC6WGBee]fs?QR:SJ8nI[g82"

Acquire and edit the code with this info. Running the file will then give you a token after logging in with the through your SSO of your company. Now this token is put in a public file, anyone who has acces to this token could theoretically scoop your email. It is therefore crucial to setup a password manager to protect this secret.

Email receiving

I use mbsync for receiving emails and mstmp for sending emails. I again defer to the respective tutorials for setting these up, but I will mention what is important for XOAUTH2. A common setup for mbsync could look something like this

#file: ~/.mbsyncrc
# mbsyncrc based on
IMAPAccount office365
# Address to connect to
PassCmd "gpg2 -q --for-your-eyes-only --no-tty -d ~/.emacs.d/mu4e/.mbsyncpass-<EMAIL_HERE>.gpg"
AuthMechs LOGIN
SSLVersions TLSv1.3
CertificateFile /etc/ssl/certs/ca-certificates.crt

IMAPStore office365
Account office365

MaildirStore office365-local
SubFolders Verbatim
# The trailing "/" is important
Path ~/Mail/
Inbox ~/Mail/office365/inbox

Channel office365
Far :office365-remote:
Near :office365-local:
# Exclude everything under the internal [Gmail] folder, except the interesting folders
Patterns * ![office365]* "[office365]/Sent Mail" "[office365]/Starred" "[office365]/All Mail"
# Or include everything
#Patterns *
# Automatically create missing mailboxes, both locally and on the server
Create Both
# Save the synchronization state files in the relevant directory
SyncState *

PassCmd is used to obtain your client secret setup above and Authmechs needs to be set to XOAUTH2 for office365 but oddly to LOGIN for gmail. One could also fill in a bare string password to prevent the hassle of setting up password managers, or use python TOKENFILE to acquire the secret but this is not recommended. We can check whether it works by running mbsync -a from your favorite shell.

Email sending

Similar to the setup of mbsync, msmtp can be setup to use a password command to insert the token. Note that you need to have a version that supports xoauth2, on AUR you can use mstmp-oauth2. A typical setup looks like this

#file : ~/.mstmprc 
# Set default values for all following accounts.
auth           on
tls            on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
logfile        ~/.msmtp.log

# Outlook
account        outlook
port           587
auth           xoauth2
# not necessary
# user           username
# not recommended
password       plain-text-password
# recommended
passwordeval   <Password retrieval command>

# Set a default account (optional)
account default : outlook

We can now receive and send email. As coup de grâce we have to setup mu4e to start using email from emacs.


At the time of writing I am using doom emacs which has some wrappers for common long commands; substitute for default emacs accordingly. “Modern” mu4e uses contexts to setup various emails addresses. I will highlight the relevant parts for mu4e setup to make our setup work. In short, we need to tell mu use mbsync to retrieve emails and msmtp to send emails.

(after! mu4e
   ;; set correct path
   sendmail-program "/usr/sbin/msmtp"
   mu4e-root-maildir "~/Mail"
   mu4e-mu4e-mail-path "~/Mail"
   send-mail-function  'smtpmail-send-it
   ;; remove adding username --> msmtp takes care of this
   message-sendmail-f-is-evil t
   ;; read who is sending the email 
   message-sendmail-extra-arguments '("--read-envelope-from")
     :name  "office365"
     :match-func (lambda (msg)
                   (when msg
                     (string-prefix-p "/office365" (mu4e-message-field msg :maildir)
     :enter-func (lambda () (mu4e-message "Entering work contenxt"))
     ;; relevant bits
     :vars '(
             (mu4e-sent-folder . "/office365/Sent Items")
             (mu4e-drafts-folder . "/office365/Drafts")
             (mu4e-inbox-folder . "/office365/inbox")
             (smtpmail-local-domain . "office365")
             (smtpmail-smtp-server . "")
             (smtpmail-default-smtp-server . "")
             (user-mail-address . "<user email address")
             ;; name setup in mbsync
             (mu4e-get-mail-command . "mbsync office365")

As a final step, we need to setup mu itself. From your favorite shell write:

mu init --maildir ~/Mail --my-address=<User address> && mu index

…and we are done! You can now use mu4e to send and receive emails from emacs, happy hacking 😉!

Casper van Elteren
Casper van Elteren
Computational scientist | Data scientist | Tinkerer

I am a computational scientist interested in data analysis, visualization and software engineering.