create mail server,postfix dovecot

https://www.atlantic.net/vps-hosting/how-to-set-up-fully-featured-mail-server-with-postfix-dovecot-and-roundcube-on-ubuntu-18-04/

https://www.vultr.com/docs/how-to-install-postfix-dovecot-and-roundcube-on-ubuntu-20-04

https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-on-ubuntu-18-04

https://linuxhostsupport.com/blog/set-up-a-mail-server-with-postfixadmin-on-ubuntu-18-04/

最新ubuntu搭建公网个人邮件服务器(基于postfix，dovecot,mysql)

https://www.digitalocean.com/community/tutorials/how-to-secure-apache-with-let-s-encrypt-on-ubuntu-18-04