Beginning this morning around 3:30AM, the virus scanner servers stopped receiving emails. The problem was discovered around 10:30 AM this morning and has been fixed.
It will take another hour or so for us to resend all the queued emails in our system. For the emails that were rejected, remote computers are usually configured to retry the sending of email and only give up after a day or two at the earliest. So no emails should be lost. The sender may have received a warning that mail could not be delivered after 4 hours though.