Attachment Monitoring and Phishing Prevention
Third party platforms can scan email in transit for malicious attachments or pre-listed banned file formats. They can even deeply scan the URLs that come through for content and context making sure they're not a Phishing attempt. Lowering the threat risks to your staff.
Most of these platforms will apply banners to the email so that the users can stay informed about the contextual nature of the threat(s), if any, and report any that might be false positives.