sudo apt-get install alpine postfix bsd-mailx
sudo vi /etc/postfix/main.cf
#=> correct "myhostname" if necessary
#=> if it doesn't exist, add the line "smtp_tls_security_level = may"
#=> if this option or the option "smtp_tls_CApath" doesn't exist,
#   add the line "smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt"
sudo dpkg-reconfigure postfix  # and configure as appropriate
echo "root: pi" | sudo tee -a /etc/aliases && echo "---" && cat /etc/aliases
sudo newaliases && sudo systemctl restart postfix
echo "This is a mailx test" | mailx -s "mailx test" root
alpine
# Configure "User Domain" and anything else as needed