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

The newly introduced "acme-mailcow" container (21st of June) will try to obtain a valid LE certificate for you.


mailcow must be available on port 80 for the acme-client to work.

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 your servers IPv4 address. If it succeeds, these names will be added as SANs to the certificate request.

You can skip the IP verification by adding SKIP_IP_CHECK=y to 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.

You could add an A record for "autodiscover" but omit "autoconfig", the client will only validate "autodiscover" and skip "autoconfig" then.

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.

Additional domain names

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


Make sure you are using acme-mailcow:1.5 or above in docker-compose.yml - if not, update mailcow first!

Do not use quotes (")!,,,

Each name will be validated against its IPv4 address.

Run docker-compose up -d to recreate changed containers.

Skip Let's Encrypt function

Change SKIP_LETS_ENCRYPT=y in mailcow.conf and restart the stack by running docker-compose down && docker-compose up -d.

Use own certificates

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.

Restart changed containers by running docker-compose up -d.

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 dovecot or postfix we will use openssl:

# Connect via SMTP (25)
openssl s_client -starttls smtp -crlf -connect
# Connect via SMTPS (465)
openssl s_client -showcerts -connect
# Connect via SUBMISSION (587)
openssl s_client -starttls smtp -crlf -connect