I am running this tutorial on Kali Linux, I will start by first installing theHarvester tool. On recent versions of Kali Linux, the tool should be already pre-installed but incase it is not run the following command to install it
sudo apt install theharvester
This is a simple program that gathers subdomain names, email addresses, virtual hosts, open ports/banners and employee names from different public sources. It can search from various sources.
In the command above, the target is cyberacademy.ac.zm, I then limit the number of results to 10 and the sources is all so that it searches through every source it has.
When you look at the result, it shows various thins such as the autonomous systems the domain name is listed in, the interesting links on the website, the IP address, there is one email address shown and a few other hosts.
To get more options you can run the following command to list all the possible options
theHarvester -h
From the result that comes, you will see that there are many sources but by using "-b all" you can search through everything. There are several options including the use of shodan to query discovered hosts, using proxies and so on. Try according to your needs.