The VERIFY command allows query of an account existence from the the mail transport protocol. In general, you do not want outsider to probe your server like this.
The EXPN command expands an address. If the address corresponds to a mailing list, it will present all the members of the list. In general, you do not want outsider to probe your server like this.
The ETRN command lets anyone on the net request sendmail to run its queue for a specific domain. This is often used by mail server with intermitent connection. They send an "ETRN domain" request when they get online.
This allows the "return receipt" messages.