Enumerating SMTP

SMTP is a protocol used to transfer messages and is commonly used in both mail servers and mail clients. The simplicity and reliability of the protocol has led to its widespread adoption and a handful of revisions since it was first introduced way back in 1982.

SMTP, in the context of a messaging system, is used two different ways. In the case of mail servers, the protocol is used to transfer messages from one server to another until it reaches the server where the recipient's mailbox is located where it is  stored for later retrieval. On the mail client side, the protocol is used to send messages to a mail server and make use of other protocols for retrieving messages from the server.

For pentesters, SMTP can represent a valuable source of information, in particular, user names and email addresses. The technique we will use here is designed to query an SMTP service and retrieve usernames along with domain names. You may not consciously think about it, but you see this information...