Currently using ethereal.email service requires non-released version of colombe and you need to checkout the project, commit edf757c58fce58c170c63e8a92d3bc81fe4d32ff contains the needed fix. Then the version with the fix needs to be pinned in the build env:
cd /path/to/colombe
opam switch /path/to/letters
# Follow instructions to evaluate the environment
git switch --detach edf757c58fce58c170c63e8a92d3bc81fe4d32ff
opam pin .
# Finally, return back to letters project
cd /path/to/letters
Then execute the tests
dune test
And finally review that the email is correctly generated in the service:
login to https://ethereal.email/login using credentials from the ethereal_account.json
check the content of messages: https://ethereal.email/messages
Credits
This project is build on colombe and mrmime libraries and use facteur as starting point.