I run Sympl on a Mythic Beasts VM as my mail server and one of my web hosts. My exim filter file had grown into a massive (776 lines), repetitive file over at least 15 years (I have git commits going back 10 years) if not longer that certainly needed a refactor, but I was worried about breaking at least my filtering if not my mail delivery entirely.

To solve this, I built with help from gemini-cli a Docker-based testing suite to verify my rules before making changes. Since its much better to have tests before a refactor. Also using docker means I can test locally before pushing the updated filter to my server.

The Testing Strategy

The system uses exim4-daemon-light inside a container and leverages exim’s filter testing mode (exim -bF). I wrote a simple bash helper to pipe mock emails into exim and verify the output:

run_test 'Spam Domain Block' \
"From: info@spamsite.best
Subject: Urgent" \
"Save message to: /path/to/Maildir/.caughtspam/"

Refactoring with Gemini CLI

With a solid test suite in place, I used gemini-cli to handle the tedious bit. It helped me:

• Consolidate lots of repetitive if blocks into cleaner or conditions.
• Standardize header checks across the entire file.
• Generate new test cases for edge cases I hadn’t covered.

The combination of a local test suite and an AI-powered CLI turned a slightly risky manual chore into a safe, automated workflow. Now I can refactor with confidence, knowing my mail is still going where it should.

As of today the filter file is down to 492 lines.

Next I plan to set up some CI to run on my recently deployed gitea instance to automatically run the tests on any future changes to the filter file.