SMTP_HOST variable empty

[theniwo]

If I set the variable within the docker-compose.yml or via .env file, it shows empty in the container and can not be set. However it is set in the config.txt

root@a17b5d5dfb37:/root# echo $SMTP_HOST

root@a17b5d5dfb37:/root# SMPT_HOST=test
root@a17b5d5dfb37:/root# echo $SMTP_HOST

root@a17b5d5dfb37:/root# grep SMTP_HOST /config/config.txt
SMTP_HOST=127.0.0.1

Also the MAIL_SENDER variable seems buggy too

root@a17b5d5dfb37:/root# echo $MAIL_SENDER
[email protected]
root@a17b5d5dfb37:/root# grep MAIL_SENDER /config/config.txt
[email protected]

Deleting the config.txt in my docker volume does not change this. Editing the file and entering correct values does not send any mail. The content is printed to the container STDOUT however.