@tomw the concurrency is set quite low at the moment - just 5. There's no issue bumping this value, it was just a very conservative setting.
As a temporary fix:
docker exec -ti mail /bin/bash
- Edit the file
/run/haraka/config/limit.ini. There is a
concurrencysection under which
maxis set to 5. Make is say 200.
supervisorctl restart haraka
Does that make things work? The configuration does not survive server restarts, but I will bump it in the next release.