This is a simple smtp server that accepts authentication
docker build -t kmarji/smtp-server .
docker run -d --rm \
--name postfix-relay \
-p 25:25 \
-e SMTP_USER=user \
-e SMTP_PASSWORD=pass \
-e MYHOSTNAME=mail.your-domain.com \
-e MYDOMAIN=your-domain.com \
-e MYNETWORKS="127.0.0.0/8 192.168.1.0/24" \
kmarji/smtp-server