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Your three Docker machines will act as a single swarm, each running a container configured to perform SPF checks on incoming mail. They will also use Postfix’s built in restrictions to filter out malformed mail, or mail which does not properly follow SMTP protocols (e.g. not starting a connection with HELO). Any mail that passes these checks will be '''relayed''' on to the actual mail server running on your host (which must be configured to accept mail addressed to users in your domain).
'''Note:''' Do NOT run use your host as the Docker Swarm manager, as that would interfere with its own
How you create the image(s) required is up to you. You may build on existing images if you wish. (tozd/postfix is relatively easy to use).
=== Bonus Marks ===
* 10%: Write an Ansible playbook that will handle the Docker installation.
* 10%: Create a replicated GlusterFS volume using all three VMs and direct
postfix (and/or any other mail service you're using) to store received messages inside.