Hemen,
MBytes should not conflict with ClamAV however I would recommend disabling ClamAV to see if the issue persists. There may be a file locking condition that's occurring. During the time of delivery, if you review the delivery logs are you seeing any GRP files being locked ?
Von See
Technical Support Supervisor
SmarterTools Inc.
(877) 357-6278
www.smartertools.com