SynopsisIt is possible to enumerate the names of valid users on the remote host.
DescriptionThe remote SMTP server answers to the EXPN and/or VRFY commands.
The EXPN command can be used to find the delivery address of mail aliases, or even the full name of the recipients, and the VRFY command may be used to check the validity of an account.
Your mailer should not allow remote users to use any of these commands, because it gives them too much information.
SolutionIf you are using Sendmail, add the option :
File Name: sendmail_expn.nasl
Exploit Ease: Exploits are available
Vulnerability Publication Date: 8/1/1982
Metasploit (SMTP User Enumeration Utility)