How to Install and Configure Postfix on Ubuntu 20.04

The server must have a static public IP address. You will also need a domain name. Let's first configure the necessary DNS records.
For the mail server, a 3rd level subdomain is usually used. As an example, we will use mail.domain-name.com. Replace the domain-name.com with your domain name. The 3rd level domain name can be arbitrary. The main thing is to use the appropriate option in further configuration. DNS records can usually be configured in the control panel of a domain name registrar or on your DNS server.
Add a DNS A record for your domain named mail and "target" equal to the IP address of the future mail server.

Add DNS MX record for the root domain with a value equal to the domain name of the mail server:

If the service returns an error as a result, try not to use nothing instead of @.

With the selected 3rd level domain name mail.domain-name.com, the hostname will be mail. Let’s configure it.

Now open the hosts file.

Let's add a line for this server with its IP address, your domain name and hostname. This entry must be placed directly below the entry for localhost.

Let’s install Postfix and mail package mailutils.

During installation, you will be prompted for options interactively. Set mail server configuration type to Internet Site.

System mail name must match the domain name for which we are setting up the mail server. In our case, this is domain-name.com.

Now the Postfix mail server is ready to work. Let’s test it and send our first email. Put your mail to the following command and run it:

Due to the fact that encryption and things like DKIM are not yet configured, your letter will most likely end up in the Spam folder. Some mail servers like gmail.com won't accept it at all. We'll fix this later.
Now you can reply to this letter in your mailbox and see this answer on the mail server.

Output:

Enter 1 and you will see your reply.