Postfix
- https://news.ycombinator.com/item?id=16538851
- https://www.tecmint.com/best-mail-transfer-agents-mta-for-linux/
- https://easyengine.io/tutorials/linux/ubuntu-postfix-gmail-smtp/
- https://blog.mailtrap.io/postfix-sendmail-exim/
- https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-on-ubuntu-20-04