Real-time Scan is a persistent and ongoing scan.
Each time a file is opened, downloaded, copied, or modified, Real-time Scan in the Security Agent scans the file for threats. For details on configuring Real-time Scan, see Configuring Real-time Scan for Security Agents.
In the case of email messages, Real-time Scan in the Messaging Security Agent (Advanced only) guards all known virus entry points by scanning all incoming messages, SMTP messages, documents posted on public folders, and files replicated from other Microsoft Exchange servers. For details on configuring Real-time Scan, see Configuring Real-time Scan for Messaging Security Agents.