Full Antivirus Intelligent Prescreening
By default, intelligent prescreening is enabled to improve antivirus scanning performance. The antivirus module generally begins to scan data after the gateway device has received all the packets of a file. Intelligent prescreening tells the antivirus module to begin scanning a file much earlier. In this case, the scan engine uses the first packet or the first several packets to determine if a file could possibly contain malicious code. The scan engine does a quick check on these first packets and if it finds that it is unlikely that the file is infected, it then decides that it is safe to bypass the normal scanning procedure.
Note: Intelligent prescreening is only intended for use with non-encoded traffic. It is not applicable for MIME encoded traffic, mail protocols (SMTP, POP3, IMAP) and HTTP POST.