3. SMTP features

3.1 Enable the VERIFY SMTP command

The VERIFY command allows the querying of an account to determine existence from the the mail transport protocol. In general, you do not want outsiders to probe your server like this.

3.2 Enable the EXPN SMTP command

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 outsiders to probe your server like this.

3.3 Enable the ETRN SMTP command

The ETRN command lets anyone on the net request sendmail to run its queue for a specific domain. This is often used by mail servers with intermitent or on-demand connections. They send an "ETRN domain" request when they get online.

3.4 Enable the VERB SMTP command

3.5 Return success DNSs21

This allows the "return receipt" messages.

3.6 Allow most SMTP status query