Port 25 - SMTP

SMTP Enumeration using nmap

enumerate smtp users

nmap -p 25 --script=smtp-enum-users 192.168.18.11

Enumerate smtp relays on target

nmap -p 25 --script smtp-open-relay 192.168.18.110

Enumerate smtp commands

nmap -p 25 --script smtp-commands 192.168.18.110

SMTP enumeration with metasploit

auxiliary/scanner/smtp/smtp_version 

auxiliary/scanner/smtp/smtp_enum
set USER_FILE /usr/share/wordlists/SecLists/Usernames/top-usernames-shortlist.txt
run