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Advanced SSL

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 address 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 the following command:

docker compose restart acme-mailcow
docker-compose restart acme-mailcow

Monitor its logs with:

docker compose logs --tail=200 -f acme-mailcow
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 (") and do not use spaces between the names!


Each name will be validated against its IPv6 address 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 the following command to recreate affected containers automatically:

docker compose up -d
docker-compose up -d


Using names other name MAILCOW_HOSTNAME to access the mailcow UI may need further configuration.

If you plan to use a server name that is not MAILCOW_HOSTNAME to access the mailcow UI (for example by adding mail.* to ADDITIONAL_SAN make sure to populate that name in mailcow.conf via ADDITIONAL_SERVER_NAMES. Names must be separated by commas and must not contain spaces. If you skip this step, mailcow may respond with an incorrect site.


Run the command below to apply:

docker compose up -d
docker-compose up -d

Force renewal

To force a renewal, you need to create a file named force_renew and restart the acme-mailcow container:

cd /opt/mailcow-dockerized
touch data/assets/ssl/force_renew
docker compose restart acme-mailcow
# Now check the logs for a renewal
docker compose logs --tail=200 -f acme-mailcow
cd /opt/mailcow-dockerized
touch data/assets/ssl/force_renew
docker-compose restart acme-mailcow
# Now check the logs for a renewal
docker-compose logs --tail=200 -f acme-mailcow

The file will be deleted 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 address. 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 address 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, 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. In most cases, the HTTP verification is skipped to workaround unknown NAT reflection issues, which are not resolved by ignoring this specific network misconfiguration. If you encounter problems generating TLSA records in the DNS overview within mailcow, you are most likely having issues with NAT reflection you should fix.

If you changed a SKIP_* parameter, run the command below to apply your changes:

docker compose up -d
docker-compose up -d

Disable Let's Encrypt

Disable Let's Encrypt completely

Set SKIP_LETS_ENCRYPT=y in "mailcow.conf" and recreate "acme-mailcow" with:

docker compose up -d
docker-compose up -d

Skip all names but ${MAILCOW_HOSTNAME}

Add ONLY_MAILCOW_HOSTNAME=y to "mailcow.conf" and recreate "acme-mailcow" with:

docker compose up -d
docker-compose up -d

The Let's Encrypt subjectAltName limit of 100 domains

Let's Encrypt currently has a limit of 100 Domain Names per Certificate.

By default, "acme-mailcow" will create a single SAN certificate for all validated domains (see the first section and Additional domain names). This provides best compatibility but means the Let's Encrypt limit exceeds if you add too many domains to a single mailcow installation.

To solve this, you can configure ENABLE_SSL_SNI to generate:

  • A main server certificate with MAILCOW_HOSTNAME and all fully qualified domain names in the ADDITIONAL_SAN config
  • One additional certificate for each domain found in the database with autodiscover., autoconfig. and any other ADDITIONAL_SAN configured in this format (subdomain.*).
  • Limitations: A certificate name will be added as SAN to the main certificate. A separate certificate/key pair will not be generated for this format.

Postfix, Dovecot and Nginx will then serve these certificates with SNI.

Set ENABLE_SSL_SNI=y in "mailcow.conf" and recreate "acme-mailcow" with:

docker compose up -d
docker-compose up -d


Not all clients support SNI, see Dovecot documentation or Wikipedia. You should make sure these clients use the MAILCOW_HOSTNAME for secure connections if you enable this feature.

Here is an example:

  • mailcow email domains: "domain1.tld" and "domain2.tld"

The following certificates will be generated:

  •, -> this is the default certificate, all clients can connect with these domains
  • mail.domain1.tld, autoconfig.domain1.tld, autodiscover.domain1.tld -> individual certificate for domain1.tld, cannot be used by clients without SNI support
  • mail.domain2.tld, autoconfig.domain2.tld, autodiscover.domain2.tld -> individual certificate for domain2.tld, cannot be used by clients without SNI support

How to use your own certificate

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.

IMPORTANT: Do not use symbolic links! Make sure you copy the certificates and do not link them to data/assets/ssl.

Restart affected services afterwards:

docker restart $(docker ps -qaf name=postfix-mailcow)
docker restart $(docker ps -qaf name=nginx-mailcow)
docker restart $(docker ps -qaf name=dovecot-mailcow)

See Post-hook script for non-mailcow ACME clients for a full example script.

Test against staging ACME directory

Edit mailcow.conf and add LE_STAGING=y.

Run the command below to activate your changes:

docker compose up -d
docker-compose up -d

Custom directory URL

Edit mailcow.conf and add the corresponding directory URL to the new variable DIRECTORY_URL:


You cannot use LE_STAGING with DIRECTORY_URL. If both are set, only LE_STAGING is used.

Run the command below to activate your changes:

docker compose up -d
docker-compose up -d

Use CAA records with the accounturi parameter

Let's Encrypt allows restricting certificate issuance to specific account IDs via the accounturi parameter in CAA records. You can use the helper script to generate a CAA record with accounturi, which you can then enter with your DNS provider:

./helper-scripts/ --account-key data/assets/ssl/acme/account.pem --contact

Check your configuration

Run the command below to find out why a validation fails:

docker compose logs --tail=200 acme-mailcow
docker-compose logs --tail=200 acme-mailcow

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 STARTTLS (587)
openssl s_client -starttls smtp -connect MAILCOW_HOSTNAME:587 | openssl x509 -noout -text
# Connect via SMTP (465)
openssl s_client -connect MAILCOW_HOSTNAME:465 | openssl x509 -noout -text

# Connect via IMAP STARTTLS (143)
openssl s_client -starttls imap -connect MAILCOW_HOSTNAME:143 | openssl x509 -noout -text
# Connect via IMAP (993)
openssl s_client -connect MAILCOW_HOSTNAME:993 | openssl x509 -noout -text

# Connect via HTTPS (443)
openssl s_client -connect MAILCOW_HOSTNAME:443 | openssl x509 -noout -text

To validate the expiry dates as returned by openssl against MAILCOW_HOSTNAME, you are able to use our helper script:

cd /opt/mailcow-dockerized
bash helper-scripts/