How to do it...

The SMTP User Enumeration Utility auxiliary module, by default, will use the unix_users.txt file located at /usr/share/metasploit-framework/data/wordlists/, but you can specify your own. To run the module, set the target address range, the number of concurrent threads, and type run:

msf > use auxiliary/scanner/smtp/smtp_enum 
msf auxiliary(smtp_enum) > set RHOSTS 192.168.216.129
msf auxiliary(smtp_enum) > set THREADS 256
THREADS => 256
msf auxiliary(smtp_enum) > run

[*] 192.168.216.129:25 - 192.168.216.129:25 Banner: 220 metasploitable.localdomain ESMTP Postfix (Ubuntu)
[+] 192.168.216.129:25 - 192.168.216.129:25 Users found: , backup, bin, daemon, distccd, ftp, games, gnats, irc, libuuid, list, lp, mail, man, news, nobody, postgres, postmaster, proxy, service, sshd, sync, sys, syslog, user, uucp, www-data
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(smtp_enum) >

The output reveals a list of valid users for the Metasploitable 2 target.