To assist others who may encounter a similar problem, I have outlined the solution I discovered after extensive troubleshooting.
The core issue involved network connectivity failures, which I diagnosed by executing a command within the Postfix container: docker container exec -it mailcowdockerized-postfix-mailcow-1 getent hosts example.com. This command revealed that the system was exclusively resolving the domain name to IPv6 addresses, which was the root cause of the connectivity failure.
To resolve this, a multi-step approach was necessary to completely disable IPv6 functionality within the Mailcow environment. The procedure was guided by the official Mailcow documentation.
First, I modified the Docker Compose configuration by creating an override file. This file deactivates the IPv6 service and disables IPv6 for the internal network with the following directives:
services:
ipv6nat-mailcow:
restart: no
entrypoint: ["echo", "ipv6nat disabled in compose.override.yml"]
networks:
mailcow-network:
enable_ipv6: false
Subsequently, I updated the main environment configuration file by appending the line DISABLE_IPv6=y to ensure all Mailcow components were aware of the new setting.
Finally, to enforce IPv4 usage for mail delivery, I directly edited the Postfix configuration file located at data/conf/postfix/main.cf. I updated the two critical parameters to their correct values:
smtp_address_preference = ipv4
inet_protocols = ipv4
In conclusion, this comprehensive solution, which involved reconfiguring the Docker network, the application environment, and the mail server itself, successfully forced the system to use IPv4, thereby restoring full internet connectivity.