How can an SMTP server be exploited?

What will be an ideal response?

Hackers can exploit the server in two ways:* The attacker can send mail anonymously. The recipients of these e-mail messages will not be able to discover where the e-mail message came from because it will reach them anonymously through an authorized source and not directly from the hacker.* Hackers can also send the SMTP server a single e-mail with the intention of reaching hundreds, thousands, or even millions of users. This method allows an attacker to send out megabytes of spam through the server's fast connection, even if the attacker is connecting through a dial-up modem.