SMTP connection and Session Timeout

Hi, my FGT-200A firewall protection email server. if SMTP AV protection is enabled then returning error message. Outlook and Outlook express SMTP error is " Your server has unexpectedly terminated the connection" now SMTP connection is session timeout down from 300 to 10 and very speed. also for SMTP AV disabled is no problem but not virus protection and no error. BUT DANGER (smt virus activity) Why ? AV update version 6.438 are you AV signatures is bug ? Thanks