Postfix

Ref: https://news.ycombinator.com/item?id=16538851 Ref: https://www.tecmint.com/best-mail-transfer-agents-mta-for-linux/ Ref: https://easyengine.io/tutorials/linux/ubuntu-postfix-gmail-smtp/ Ref: https://blog.mailtrap.io/postfix-sendmail-exim/