Hi,
There are many really good tutorials online, eg:
PHP Code:
https://www.linuxbabe.com/mail-server/setup-basic-postfix-mail-sever-ubuntu
PHP Code:
https://www.linux.com/training-tutorials/install-and-configure-postfix-mail-server/
PHP Code:
https://www.tecmint.com/setup-postfix-mail-server-in-ubuntu-debian/
These are just the top results from a google search.