With MTA (Mail Transfer Agent) enabled the gateways manages SMTP traffic and holds external email...

 To enable MTA (Mail Transfer Agent) functionality in the Security Gateway, the Threat Prevention Software Blade Package is required. The Threat Prevention Software Blade Package includes the Anti-Virus, Anti-Bot, and Threat Emulation blades, which can scan and hold external email with potentially malicious attachments. The MTA functionality allows the Security Gateway to act as an SMTP relay between the mail server and the Internet, and apply Threat Prevention policies to the email traffic. The other options are either not related or not sufficient to enable MTA functionality. R