This is activated by ticking the box on the Malware Report page:
Our free Website Malware Scanner makes daily scans of all the sites within your hosting account. It uses a combination of commercial and in-house tools and provides reports detailing identified malicious content and its location within your site files.
When malware is located on a site, PHP mail is automatically disabled. We do this to preserve sender reputation across the platform and ensure that any sites that are compromised do not send large volumes of spam emails.