Thank you very much, @andreas-doehler, for your reply!
I agree. I don’t have too much experience with postfix. But you can run the below simple commands to reproduce the issue with the official check-mk-raw docker image. Maybe this is an issue with the image?
docker container run --rm -d -p 8080:5000 -p 8000:8000 --tmpfs /opt/omd/sites/cmk/tmp:uid=1000,gid=1000 -v /omd/sites --name monitoring -v /etc/localtime:/etc/localtime:ro -e MAIL_RELAY_HOST='mailrelay.mydomain.com' --hostname monitoring.mydomain.com checkmk/check-mk-raw:2.1.0-latest
docker exec -it monitoring bash
postfix_pid=$(sed 's/ //g' </var/spool/postfix/pid/master.pid)
readlink -- "/proc/${postfix_pid}/exe"
The issue is that the last line returns an empty string. This is why checkmk reports CRIT status for the postfix service. But the postfix service is indeed running (was started in the entry point script because the MAIL_RELAY_HOST env var was set). You can check this with service postfix status.
Any idea?