Chatmail servers for secure instant messaging

Chatmail servers are interoperable email routing machines optimized for:

• Convenience: Low friction instant onboarding

• Privacy: No name, phone numbers, email required or collected

• Instant: Privacy-preserving push notifications for Apple, Google, and Huawei

• Speed: Message delivery in well under a second.

• Security: Strict TLS, DKIM and OpenPGP with metadata-minimization rules enforced.

• Reliability: No spam or IP reputation checks, rate-limits suitable for realtime chats.

• Efficiency: Messages are only stored for transit and removed automatically.

This repository contains everything needed to setup a ready-to-use chatmail server comprised of a minimal setup of the battle-tested Postfix SMTP and Dovecot IMAP services.

The automated setup is designed and optimized for providing chatmail addresses for immediate permission-free onboarding through chat apps and bots. Chatmail addresses are automatically created at first login, after which the initially specified password is required for sending and receiving messages through them.

Please see this list of known apps and client projects which offer instant onboarding on chatmail servers and this list of known public 3rd party chatmail servers.

Minimal requirements, Prerequisites

You will need the following:

• Control over a domain through a DNS provider of your choice.

• A Debian 12 server with reachable SMTP/SUBMISSIONS/IMAPS/HTTPS ports. IPv6 is encouraged if available. Chatmail servers only require 1GB RAM, one CPU, and perhaps 10GB storage for a few thousand active chatmail addresses.

• Key-based SSH authentication to the server's root user. You must add a passphrase-protected private key to the local ssh-agent because you can't type in your passphrase during deployment. (An ed25519 private key is required due to an upstream bug in paramiko)

Getting started

We use chat.example.org as the chatmail domain in the following steps. Please substitute it with your own domain.

1. Setup the initial DNS records. The following is an example in the familiar BIND zone file format with a TTL of 1 hour (3600 seconds). Please substitute your domain and IP addresses.

    chat.example.com. 3600 IN A 198.51.100.5
    chat.example.com. 3600 IN AAAA 2001:db8::5
    www.chat.example.com. 3600 IN CNAME chat.example.com.
    mta-sts.chat.example.com. 3600 IN CNAME chat.example.com.
   

2. Clone the repository and bootstrap the Python virtualenv.

    git clone https://github.com/chatmail/server
    cd server
    scripts/initenv.sh
   

3. Create chatmail configuration file chatmail.ini:

    scripts/cmdeploy init chat.example.org  # <-- use your domain 
   

4. Verify that SSH root login works:

    ssh root@chat.example.org   # <-- use your domain 
   

5. Deploy to the remote chatmail server:

    scripts/cmdeploy run
   

   This script will check that you have all necessary DNS records. If DNS records are missing, it will recommend which you should configure at your DNS provider (it can take some time until they are public).

Other helpful commands:

To check the status of your remotely running chatmail service:

scripts/cmdeploy status

To display and check all recommended DNS records:

scripts/cmdeploy dns

To test whether your chatmail service is working correctly:

scripts/cmdeploy test

To measure the performance of your chatmail service:

scripts/cmdeploy bench

Overview of this repository

This repository has four directories:

• cmdeploy is a collection of configuration files and a pyinfra-based deployment script.

• chatmaild is a Python package containing several small services which handle authentication, trigger push notifications on new messages, ensure that outbound mails are encrypted, delete inactive users, and some other minor things. chatmaild can also be installed as a stand-alone Python package.

• www contains the html, css, and markdown files which make up a chatmail server's web page. Edit them before deploying to make your chatmail server stand out.

• scripts offers two convenience tools for beginners; initenv.sh installs the necessary dependencies to a local virtual environment, and the scripts/cmdeploy script enables you to run the cmdeploy command line tool in the local virtual environment.

cmdeploy

The cmdeploy/src/cmdeploy/cmdeploy.py command line tool helps with setting up and managing the chatmail service. cmdeploy init creates the chatmail.ini config file. cmdeploy run uses a pyinfra-based script to automatically install or upgrade all chatmail components on a server, according to the chatmail.ini config.

The components of chatmail are:

• Postfix SMTP server accepts sent messages (both from your users and from other servers)

• Dovecot IMAP server stores messages for your users until they download them

• Nginx shows the web page with your privacy policy and additional information

• acmetool manages TLS certificates for Dovecot, Postfix, and Nginx

• OpenDKIM for signing messages with DKIM and rejecting inbound messages without DKIM

• mtail for collecting anonymized metrics in case you have monitoring

• and the chatmaild services, explained in the next section:

chatmaild

chatmaild offers several commands which differentiate a chatmail server from a classic mail server. If you deploy them with cmdeploy, they are run by systemd services in the background. A short overview:

• doveauth implements create-on-login account creation semantics and is used by Dovecot during login authentication and by Postfix which in turn uses Dovecot SASL to authenticate users to send mails for them.

• filtermail prevents unencrypted email from leaving the chatmail service and is integrated into Postfix's outbound mail pipelines.

• chatmail-metadata is contacted by a dovecot lua script to store user-specific server-side config. On new messages, it passes the user's push notification token to notifications.delta.chat so the push notifications on the user's phone can be triggered by Apple/Google/Huawei.

• delete_inactive_users deletes users if they have not logged in for a very long time. The timeframe can be configured in chatmail.ini.

• lastlogin is contacted by dovecot when a user logs in and stores the date of the login.

• echobot is a small bot for test purposes. It simply echoes back messages from users.

• chatmail-metrics collects some metrics and displays them at https://example.org/metrics.

Home page and getting started for users

cmdeploy run also creates default static web pages and deploys them to a Nginx web server with:

• a default index.html along with a QR code that users can click to create accounts on your chatmail provider

• a default info.html that is linked from the home page

• a default policy.html that is linked from the home page

All .html files are generated by the according markdown .md file in the www/src directory.

Refining the web pages

scripts/cmdeploy webdev

This starts a local live development cycle for chatmail web pages:

• uses the www/src/page-layout.html file for producing static HTML pages from www/src/*.md files

• continously builds the web presence reading files from www/src directory and generating HTML files and copying assets to the www/build directory.

• Starts a browser window automatically where you can "refresh" as needed.

Emergency Commands to disable automatic account creation

If you need to stop account creation, e.g. because some script is wildly creating accounts, login to the server with ssh and run:

    touch /etc/chatmail-nocreate

Account creation will be denied while this file is present.

Ports

Postfix listens on ports 25 (SMTP) and 587 (SUBMISSION) and 465 (SUBMISSIONS). Dovecot listens on ports 143 (IMAP) and 993 (IMAPS). Nginx listens on port 8443 (HTTPS-ALT) and 443 (HTTPS). Port 443 multiplexes HTTPS, IMAP and SMTP using ALPN to redirect connections to ports 8443, 465 or 993. acmetool listens on port 80 (HTTP).

chatmail-core based apps will, however, discover all ports and configurations automatically by reading the autoconfig XML file from the chatmail server.

Email authentication

chatmail servers rely on DKIM to authenticate incoming emails. Incoming emails must have a valid DKIM signature with Signing Domain Identifier (SDID, d= parameter in the DKIM-Signature header) equal to the From: header domain. This property is checked by OpenDKIM screen policy script before validating the signatures. This correpsonds to strict DMARC alignment (adkim=s), but chatmail does not rely on DMARC and does not consult the sender policy published in DMARC records. Other legacy authentication mechanisms such as iprev and SPF are also not taken into account. If there is no valid DKIM signature on the incoming email, the sender receives a "5.7.1 No valid DKIM signature found" error.

Outgoing emails must be sent over authenticated connection with envelope MAIL FROM (return path) corresponding to the login. This is ensured by Postfix which maps login username to MAIL FROM with smtpd_sender_login_maps and rejects incorrectly authenticated emails with reject_sender_login_mismatch policy. From: header must correspond to envelope MAIL FROM, this is ensured by filtermail proxy.

TLS requirements

Postfix is configured to require valid TLS by setting smtp_tls_security_level to verify. If emails don't arrive from a chatmail server to your server, the problem is likely that your server does not have a valid TLS certificate.

You can test it by resolving MX records of your server domain and then connecting to MX servers (e.g mx.example.org) with openssl s_client -connect mx.example.org:25 -verify_hostname mx.example.org -verify_return_error -starttls smtp from the host that has open port 25 to verify that certificate is valid.

When providing a TLS certificate to your server, make sure to provide the full certificate chain and not just the last certificate.

If you are running Exim server and don't see incoming connections from a chatmail server in the logs, make sure smtp_no_mail log item is enabled in the config with log_selector = +smtp_no_mail. By default Exim does not log sessions that are closed before sending the MAIL command. This happens if certificate is not recognized as valid by Postfix, so you might think that connection is not established while actually it is a problem with your TLS certificate.

Migrating chatmail server to a new host

If you want to migrate chatmail from an old machine to a new machine, you can use these steps. They were tested with a Linux laptop; you might need to adjust some of the steps to your environment.

Let's assume that your mail_domain is mail.example.org, all involved machines run Debian 12, your old server's IP address is 13.37.13.37, and your new server's IP address is 13.12.23.42.

Note, you should lower the TTLs of your DNS records to a value such as 300 (5 minutes) so the migration happens as smoothly as possible.

During the guide you might get a warning about changed SSH Host keys; in this case, just run ssh-keygen -R "mail.example.org" as recommended.

1. First, disable mail services on the old server.

    cmdeploy run --disable-mail --ssh-host 13.37.13.37
   

   Now your users will notice the migration and will not be able to send or receive messages until the migration is completed.

2. Now we want to copy /home/vmail, /var/lib/acme, /etc/dkimkeys, /run/echobot, and /var/spool/postfix to the new server. Login to the old server while forwarding your SSH agent so you can copy directly from the old server to the new server with your SSH key:

    ssh -A root@13.37.13.37
    tar c - /home/vmail/mail /var/lib/acme /etc/dkimkeys /run/echobot /var/spool/postfix | ssh root@13.12.23.42 "tar x -C /"
   

   This transfers all accounts, the TLS certificate, DKIM keys (so DKIM DNS record remains valid), and the echobot's password so it continues to function. It also preserves the Postfix mail spool so any messages pending delivery will still be delivered.

3. Install chatmail on the new machine:

    cmdeploy run --disable-mail --ssh-host 13.12.23.42
   

   Postfix and Dovecot are disabled for now; we will enable them later. This ensures all user accounts are provisioned.

4. On the new server, run the following to ensure the ownership is correct in case UIDs/GIDs changed:

    chown root: -R /var/lib/acme
    chown opendkim: -R /etc/dkimkeys
    chown vmail: -R /home/vmail/mail
    chown echobot: -R /run/echobot
   

5. Now, update DNS to the new IP addresses.

   If other servers try to deliver messages to your server they may fail, but normally email servers will retry delivering messages for at least a week, so messages will not be lost.

6. Finally, you can execute cmdeploy run --ssh-host 13.12.23.42 to turn on chatmail on the new server. Your users will be able to use the chatmail server as soon as the DNS changes have propagated. Voilà!

Setting up a reverse proxy

A chatmail server does not depend on the client IP address for its operation, so it can be run behind a reverse proxy. This will not even affect incoming mail authentication as DKIM only checks the cryptographic signature of the message and does not use the IP address as the input.

For example, you may want to self-host your chatmail server and only use hosted VPS to provide a public IP address for client connections and incoming mail. You can connect chatmail server to VPS using a tunnel protocol such as WireGuard and setup a reverse proxy on a VPS to forward connections to the chatmail server over the tunnel. You can also setup multiple reverse proxies for your chatmail server in different networks to ensure your server is reachable even when one of the IPs becomes inaccessible due to hosting or routing problems.

Note that your server still needs to be able to make outgoing connections on port 25 to send messages outside.

To setup a reverse proxy (or rather Destination NAT, DNAT) for your chatmail server, put the following configuration in /etc/nftables.conf:

#!/usr/sbin/nft -f

flush ruleset

define wan = eth0

# Which ports to proxy.
#
# Note that SSH is not proxied
# so it is possible to log into the proxy server
# and not the original one.
define ports = { smtp, http, https, imap, imaps, submission, submissions }

# The host we want to proxy to.
define ipv4_address = AAA.BBB.CCC.DDD
define ipv6_address = [XXX::1]

table ip nat {
        chain prerouting {
                type nat hook prerouting priority dstnat; policy accept;
                iif $wan tcp dport $ports dnat to $ipv4_address
        }

        chain postrouting {
                type nat hook postrouting priority 0;

                oifname $wan masquerade
        }
}

table ip6 nat {
        chain prerouting {
                type nat hook prerouting priority dstnat; policy accept;
                iif $wan tcp dport $ports dnat to $ipv6_address
        }

        chain postrouting {
                type nat hook postrouting priority 0;

                oifname $wan masquerade
        }
}

table inet filter {
        chain input {
                type filter hook input priority filter; policy drop;

                # Accept ICMP.
                # It is especially important to accept ICMPv6 ND messages,
                # otherwise IPv6 connectivity breaks.
                icmp type { echo-request } accept
                icmpv6 type { echo-request, nd-neighbor-solicit, nd-router-advert, nd-neighbor-advert } accept

                # Allow incoming SSH connections.
                tcp dport { ssh } accept

                ct state established accept
        }
        chain forward {
                type filter hook forward priority filter; policy drop;

                ct state established accept
                ip daddr $ipv4_address counter accept
                ip6 daddr $ipv6_address counter accept
        }
        chain output {
                type filter hook output priority filter;
        }
}

Run systemctl enable nftables.service to ensure configuration is reloaded when the proxy server reboots.

Uncomment in /etc/sysctl.conf the following two lines:

net.ipv4.ip_forward=1
net.ipv6.conf.all.forwarding=1

Then reboot the server or do sysctl -p and nft -f /etc/nftables.conf.

Once proxy server is set up, you can add its IP address to the DNS.