Common Things With Docker Mailserver • Obsidian Garden

1. docker-mailserver SSL alert number 42

I was getting following error after setting up TLS certificates in docker-mailserver after setting it up with letsencrypt:

TLS handshaking: SSL_accept() failed: error:14094412:SSL routines:ssl3_read_bytes:sslv3 alert bad certificate: SSL alert number 42

The problem was caused by two reasons

1.1. Letsencrypt and wildcard domains

My MX domain was example.com, bu letsencrypt have been set up with wildcard certificate on *.example.com.

So, specifying SSL_MODE=letsencrypt made it searching for /etc/letsencrypt/mail.example.com, when it was just /etc/letsencrypt/example.com

SOLUTION:

Change .env file to:

SSL_MODE=manual
SSL_KEY_PATH=/etc/letsencrypt/example.com/fullchain.pem
SSL_KEY_PATH=/etc/letsencrypt/example.com/privkey.pem

Don't forget to modify docker-compose volumes and pass certs from host filesystem. Note: they're not mail.example.com, they're just example.com.

This ticket describes it well.

1.2. Docker was using internal hostname of docker-mailserver

Internal hostname for docker-mailserver was just mail, and other container tried to send emails to mail:25.

SOLUTION: change internal hostname in your app settings to actual, specified in your certificates and MX record.

2. docker-mailserver not listening on 25 port (SMTP)

Solution was to add quotes in your docker compose, just like that:

  ports:    - "25:25"

This ticket solved it

3. Connect to docker-mailserver from other docker-compose

I've had mailserver/compose.yaml and application/compose.yaml and needed to connect application to mailserver.

// application/compose.yamlapp:  networks:    - sharednetworks:  shared:    driver: bridge

/// mailserver/compose.yamlmail:  networks:     - "application_shared"networks:  application_shared:    external: true

4. How to convert DKIM mail.txt to DNS record

My mail.txt was like:

mail._domainkey IN      TXT     ( "v=DKIM1; h=sha256; k=rsa; "
          "p=sOmEJuNkLiKeRaNdOmOrLikeThat"
          "eVeNmOrERANdOmStuFf" )  ; ----- DKIM key mail for example.com

SOLUTION: concatenate all strings inside round brackets:

v=DKIM1; h=sha256; k=rsa; p=sOmEJuNkLiKeRaNdOmOrLikeThateVeNmOrERANdOmStuFf

And add them as a TXT record on my DNS provider control panel under mail._domainkey record.

Solution was right in official documentation, just hidden a little bit.