25/tcp open smtp Postfix smtpd – User enumeration

nmap

nmap is great

if you will find by nmap 25/tcp open smtp Postfix smtpd

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

you can play with this port

> 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