SMTP

nmap --script=smtp-commands,smtp-enum-users,smtp-vuln-cve2010-4344,smtp-vuln-cve2011-1720,smtp-vuln-cve2011-1764 -p 25 192.168.X.X

#USER ENUM
smtp-user-enum -M VRFY -U /usr/share/wordlists/metasploit/unix_users.txt -t $ip

# For multiple servers
for server in $(cat smtpmachines); do echo "******************" $server "*****************"; smtp-user-enum -M VRFY -U userlist.txt -t $server;done #for multiple servers

Connect to

telnet $ip 110
USER user
PASS pass
LIST
RETR 1