Proton Mail is a great web email service. If you've landed here, you probably already agree with that. However, one major missing feature is SMTP/IMAP access to your mailboxes. They offer a "local" bridge application which will communicate with their backend and then provide a local SMTP/IMAP service listening on some seemingly arbitrary ports.

Challenges

So there are a few challenges here that we've got to overcome:

1. Application by default requires a GUI to login, run as a service, etc.
2. Application listens only on localhost (127.0.0.1)
3. And only on non-standard ports

Prerequisites

So first, lets install some prerequisites.

1. pass
2. gnupg
3. Clone the proton-bridge repository - Available at https://github.com/ProtonMail/proton-bridge

Next, lets figure out how to start the thing on a server without a desktop environment.

First, you'll want to setup a passphrase-free GPG key.

$ gpg --batch --passphrase '' --quick-gen-key 'ProtonMail Bridge' default default never

This will generate a new key with the name ProtonMail Bridge in your local keyring.

Then we'll need to setup a new password database in pass.

$ pass init "ProtonMail Bridge"

Now we've got all of the prerequisite steps out of the way!

Proton-Bridge package

Finally, we can begin installing the actual ProtonMail proton-bridge package. The problem is, the *.deb (or other format) package that they provide on their download page requires a ton of graphical dependencies that we don't want to install on our server. Fortunately they have a make target called build-nogui setup in their Makefile in the repository. Before we begin compiling it with this special target, there is one change we must make in the source-code.

0.0.0.0 Host

Currently, the bridge is designed as a local application to run on your desktop machine to enable you to use Thunderbird/Outlook with ProtonMail. However, when running on a server, we often want to connect to it from the outside. So we've got to make a small change in the internal/bridge/constants.go file.

Change line 22 from Host = '127.0.0.1' to Host = '0.0.0.0'.

Note: There is an open PR to make this user configurable, but it's been open for a few months without much action. Source: https://github.com/ProtonMail/proton-bridge/pull/270

That's all thats required to change where it listens!

build-nogui Compilation

cd to where you've cloned the proton-bridge repository if you're not still there. Now we can run make build-nogui. This should compile the simple go binary proton-bridge without all of the GUI qt-* libraries and the like.

We should now have a compiled proton-bridge binary in the root of the repository which does not require a GUI to setup!

Authentication

Finally, we can begin authenticating and getting this thing up and running.

Execute the binary with the --cli flag to enter into an interactive CLI mode.

$ proton-bridge --cli

Here use the login command to begin the login procedure. This will ask you for your normal web-based ProtonMail username/password and potentially 2FA code.

After logging in successfully, you can type info in the interactive shell and it should print out a small table of information like the current host/port/username/password required to connect to both SMTP and IMAP.

We're so close!

Finally, we can set this proton-bridge to run as a service with a small custom systemd unit file.

$ sudo vim /etc/systemd/system/proton-bridge.service

And enter something like the following:

[Unit]
Description=ProtonMail Bridge
After=network.target

[Service]
User=ndo
ExecStart=/usr/local/bin/proton-bridge --noninteractive
StandardOutput=append:/var/log/proton-bridge/stdout.log
StandardError=append:/var/log/proton-bridge/stderr.log

[Install]
WantedBy=multi-user.target

Remember to adjust the ExecStart path to the binary. In my case, I've symlinked the binary from its compilation source folder to /usr/local/bin/proton-bridge. Also notice the --noninteractive flag we're passing here.

After saving and closing that file, enable and start the systemd service.

$ sudo systemctl enable proton-bridge
$ sudo systemctl start proton-bridge

This should start the proton-bridge service now and enable it to auto-start on reboot.

You can check the current status by executing sudo systemctl status proton-bridge and as the service file indicated, the logs are located at /var/log/proton-bridge/*.

Ports

Finally, after starting that service we have the IMAP and SMTP bridge running as a daemon on our server! However, it's still using the odd arbitrary ports. This may be fine if your client application allows you to manually input any number in the port field. However, some services only allow you to select from a dropdown of a preselected few IMAP ports, for example.

For cases like these, we can use iptables to our advantage!

These commands will forward:

1. 143 -> 1143
2. 25 -> 1025

$ iptables -t nat -A PREROUTING -i vmbr0 -p tcp --dport 143 -j DNAT --to-destination 10.0.1.52:1143 # imap
$ iptables -t nat -A PREROUTING -i vmbr0 -p tcp --dport 25 -j DNAT --to-destination 10.0.1.52:1025 # smtp

You can see in my case, I'm using a Proxmox host (with an incoming connection on interface vmbr0) that also needed to forward that port to a guest located at 10.0.1.52, for example. Don't forget to change that internal IP to wherever your proton-bridge service is running, or if its on the same machine just set it to 127.0.0.1 or 0.0.0.0. Also you're probably going to need to change the name of the interface and potentially the port numbers, depending on what the proton-bridge interactive info command told you earlier.

Summary

So with a bit of tweaking we were able to use the pre-existing local bridge GUI application, to run a headless server daemon to allow us to connect via SMTP / IMAP from any client to our awesome ProtonMail inboxes!