smtp-user-enum is a username guessing tool primarily for use against the default Solaris SMTP service.
Metasploit provides two SMTP auxiliary Modules i.e.,
smtp_version. Both are used for SMTP enumeration and provide adequate information about the SMTP server.
nmap nse scripts
nmap provides special scripts for SMTP enumeration.
smtp-enum-users is one of the scripts that is provided by Nmap.