https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-as-a-send-only-smtp-server-on-debian-10
https://www.makeuseof.com/postfix-mail-server-setup-on-debian/
https://wiki.debian.org/Postfix

sudo apt install mailutils
sudo apt install postfix
sudo apt remove sendmail
# REBOOT & Check to see if Postfix is running
sudo cat /var/log/mail.log
sudo vim /etc/postfix/main.cf

    You should see: 

Trying 127.0.0.1...
Connected to localhost.localdomain.
Escape character is '^]'.
220 server1.example.com ESMTP Postfix (Debian/GNU)

    Send an email to yourself: 

mail from:<you@youremail.com>
rcpt to:<user@example.com>
data
To: user@example.com
From: you@youremail.com
Subject: Hey my first email
This is my first email on debian postfix after installing configuring it.
It was easy.
    To end data hit enter, type in a dot, and hit enter again: 

.

    Then 

quit
# DNS

host -t MX yourdomain.tld

sudo vi     /etc/postfix/main.cf: 

smtpd_recipient_restrictions = reject_invalid_hostname,
        reject_unknown_recipient_domain,
        reject_unauth_destination,
        reject_rbl_client sbl.spamhaus.org,
        permit

smtpd_helo_restrictions = reject_invalid_helo_hostname,
        reject_non_fqdn_helo_hostname,
        reject_unknown_helo_hostname

smtpd_client_restrictions = reject_rbl_client dnsbl.sorbs.net


sudo dpkg-reconfigure postfix
sudo postfix reload
telnet localhost 25
telnet localhost 25