The VSE 8.5 version of virus scan installed in Exchange server

The VSE 8.5 version of virus scan is installed in our Exchange server. After enabling on access scan, we are having difficulties in sending emails. The emails are not sent.
They are simply on a queue and are accumulating on the server. If I disable the on access scan, the emails are all sent without any problem. When I installed the VSE 8.7 version, we still had the same problems. By the way, we are on EPO 4.5. Can anyone tell me what the problem is?
