Some notes regarding the installation & configuration of postfix in the most secure way I know so far today.

The target operating system is an Ubuntu 16.

sudo apt-get install postfix 
sudo apt-get install mailutils

Copy the default config:

cp /usr/share/postfix/main.cf.debian /etc/postfix/main.cf

This is my configuration for a host that it is isolated in internet, and it has no special needs (it is not the mail server for any domain, it only needs to send some emails):

# See /usr/share/postfix/main.cf.dist for a commented, more complete version


mail_owner = postfix

# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
myhostname = cloud.mister-i.com

mydomain = mister-i.com

myorigin = $myhostname

mydestination = $myhostname, localhost, novaprime

mynetworks_style = host

relay_domains = $mydestination

smtp_generic_maps = hash:/etc/postfix/generic

smtpd_banner = $myhostname ESMTP $mail_name
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no
compatibility_level = 2

inet_interfaces = all
inet_protocols = all

smtpd_use_tls = no
smtp_use_tls = no 

ufw allow 25 465 587

http://serverfault.com/questions/149903/what-ports-to-open-for-mail-server

mail --subject="hello from server" destination-address@example.com