There’s something wrong with Avast approach to email filtering. We had our Exchange failing 4 times so far to send or receive emails. Repairing Avast helped for a day. The only way was to execute on Exchange disable-transportagent “avast plugin for exchange” command and then restart Exchange Transport service. Once Avast stopped filtering, all was fine.
I don’t think Avast for Exchange is mature enough at this stage. We look after 17 Exchange servers and won’t be using it after 30 days trial. Just too many problems.