SMTP enumeration

smtp-user-enum

smtp-user-enum is a username guessing tool primarily for use against the default Solaris SMTP service.

Metasploit modules

Metasploit provides two SMTP auxiliary Modules i.e., smtp_enum and 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.