SMTP

=====================Interecting with Service : 
>>> nc -nvv $IP 25
>>> telnet $IP  25

=====================Vulnnerability Scan : 
>>> nmap --script=smtp-commands,smtp-enum-users,smtp-vuln-cve2010-4344,smtp-vuln-cve2011-1720,smtp-vuln-cve2011-1764 -p 25 $IP


=====================Always Do User Enmumeration : 
>>> smtp-user-enum -M VRFY -U /root/sectools/SecLists/Usernames/Names/names.txt -t $IP
>>> smtp-user-enum -U /usr/share/wordlists/names.txt -t $IP  -m 150
>>> smtp-user-enum -M VRFY -U /usr/share/wordlists/metasploit/unix_users.txt -t $IP
msf > use auxiliary/scanner/smtp/smtp_enum


=====================Brute Force Attack : 
>>> hydra -l root -P /root/SecLists/Passwords/10_million_password_list_top_100.txt $IP telnet

Mannually

Command to check if a user exists
>VRFY root

Command to ask the server if a user belongs to a mailing list
>EXPN root