Informational

sudo apt-get update

sudo apt-get install postfix -y

sudo apt-get install mailutils

To reconfigure postfix:
dpkg-reconfigure postfix

In order to make fake hosts in /etc/hosts working instead of leveraging MX records:
smtp_dns_support_level = disabled in /etc/postfix/main.cf.

Some MTAs even query DNS to see if FQDN in the smtpd banner resolves to the IP of your mail server.

Enter the following command to see the FQDN form of your hostname.

hostname -f

If your Ubuntu server doesn’t have an FQDN yet, you can use hostnamectl to set one.

sudo hostnamectl set-hostname your-fqdn