What useful information is gathered during a successful Simple Mail Transfer Protocol (SMTP) enumeration?

A. A list of all mail proxy server addresses used by the targeted host.
B. The internal command RCPT provides a list of ports open to message traffic.
C. The two internal commands VRFY and EXPN provide a confirmation of valid users, email addresses, aliases, and mailing lists.
D. Reveals the daily outgoing message limits before mailboxes are locked.