https://habilisbest.com/raspberry-pi-as-personal-email-server
pi@raspberrypi:~ $ nmap -p 0-65535 portquiz.net > /tmp/nmaptest
pi@raspberrypi:~ $ grep filtered /tmp/nmaptest
0/tcp filtered unknown
8963/tcp filtered unknown
18308/tcp filtered unknown
24476/tcp filtered unknown
25291/tcp filtered unknown
28165/tcp filtered unknown
35814/tcp filtered unknown
pi@raspberrypi:~ $
https://pimylifeup.com/raspberry-pi-email-server/
https://keytosmart.com/single-board-computers/running-mail-server-raspberry-pi-citadel/
https://www.thewindowsclub.com/5-free-mail-servers-for-windows-10