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Let's Encrypt (out-of-the-box)

The "acme-mailcow" container will try to obtain a LE certificate for ${MAILCOW_HOSTNAME}, autodiscover.ADDED_MAIL_DOMAIN and autoconfig.ADDED_MAIL_DOMAIN.


mailcow must be available on port 80 for the acme-client to work. Our reverse proxy example configurations do cover that. You can also use any external ACME client (certbot for example) to obtain certificates, but you will need to make sure, that they are copied to the correct location and a post-hook reloads affected containers. See more in the Reverse Proxy documentation.

By default, which means 0 domains are added to mailcow, it will try to obtain a certificate for ${MAILCOW_HOSTNAME}.

For each domain you add, it will try to resolve autodiscover.ADDED_MAIL_DOMAIN and autoconfig.ADDED_MAIL_DOMAIN to its IPv6 or - if IPv6 is not configured in your domain - IPv4 address. If it succeeds, a name will be added as SAN to the certificate request.

Only names that can be validated, will be added as SAN.

For every domain you remove, the certificate will be moved and a new certificate will be requested. It is not possible to keep domains in a certificate, when we are not able validate the challenge for those.

If you want to re-run the ACME client, use docker-compose restart acme-mailcow and monitor its logs with docker-compose logs --tail=200 -f acme-mailcow.

Additional domain names

Edit "mailcow.conf" and add a parameter ADDITIONAL_SAN like this:

Do not use quotes (")!


Each name will be validated against its IPv6 or - if IPv6 is not configured in your domain - IPv4 address.

A wildcard name like smtp.* will try to obtain a smtp.DOMAIN_NAME SAN for each domain added to mailcow.

Run docker-compose up -d to recreate affected containers automatically.

Validation errors and how to skip validation

You can skip the IP verification by setting SKIP_IP_CHECK=y in mailcow.conf (no quotes). Be warned that a misconfiguration will get you ratelimited by Let's Encrypt! This is primarily useful for multi-IP setups where the IP check would return the incorrect source IP. Due to using dynamic IPs for acme-mailcow, source NAT is not consistent over restarts.

If you encounter problems with "HTTP validation", but your IP confirmation succeeds, you are most likely using firewalld, ufw or any other firewall, that disallows connections from br-mailcow to your external interface. Both firewalld and ufw disallow this by default. It is often not enough to just stop these firewall services. You'd need to stop mailcow (docker-compose down), stop the firewall service, flush the chains and restart Docker.

You can also skip this validation method by setting SKIP_HTTP_VERIFICATION=y in "mailcow.conf". Be warned that this is discouraged. Some DNS validations (like TLSA lookups) in mailcow UI will fail.

If you changed a SKIP_* parameter, run docker-compose up -d to apply your changes.

Disable Let's Encrypt

Disable Let's Encrypt completely

Set SKIP_LETS_ENCRYPT=y in "mailcow.conf" and recreate "acme-mailcow" by running docker-compose up -d.

Skip all names but ${MAILCOW_HOSTNAME}

Add ONLY_MAILCOW_HOSTNAME=y to "mailcow.conf" and recreate "acme-mailcow" by running docker-compose up -d.

How to use your own ceritficate

Make sure you disable mailcows internal LE client (see above).

To use your own certificates, just save the combined certificate (containing the certificate and intermediate CA/CA if any) to data/assets/ssl/cert.pem and the corresponding key to data/assets/ssl/key.pem.

Reload affected service:

docker exec $(docker ps -qaf name=postfix-mailcow) postfix reload
docker exec $(docker ps -qaf name=nginx-mailcow) nginx -s reload
docker exec $(docker ps -qaf name=dovecot-mailcow) dovecot reload

Check your configuration

Run docker-compose logs acme-mailcow to find out why a validation fails.

To check if nginx serves the correct certificate, simply use a browser of your choice and check the displayed certificate.

To check the certificate served by Postfix, Dovecot and Nginx we will use openssl:

# Connect via SMTP (587)
echo "Q" | openssl s_client -starttls smtp -crlf -connect
# Connect via IMAP (143)
echo "Q" | openssl s_client -starttls imap -showcerts -connect
# Connect via HTTPS (443)
echo "Q" | openssl s_client -connect