Run exim4 in a docker, configured to send all mails to the "root"-machine (default) or somewhere else (user-config).
# get this repo
git clone git clone https://github.com/finkregh/exim4-light-docker-docker.git
# show default config
cat etc/env
# OPTIONAL: set different smarthost
#echo "export EXIM_SMARTHOST=<ip>" >> .env
# OPTIONAL: build docker
#docker build -t "finkregh/exim4-light" .
# start docker
./docker-bin/start-exim
docker run -i -t -entrypoint="/bin/sh" finkregh/exim4-light -c /bin/bash