Port 23 - smtp

nmap -n -sV -Pn -p- 192.168.0.102
...
25/tcp    open  smtp    Postfix smtpd
...

> telnet 192.168.0.102 25

> VRFY root
< 252 2.0.0 root

> VRFY szalek
< 550 5.1.1 <szalek>: Recipient address rejected: User unknown in local recipient table

> VRFY admin
< 550 5.1.1 <admin>: Recipient address rejected: User unknown in local recipient table

> VRFY user
< 252 2.0.0 user