One little problem im having is that we have users that receive encrypted email and Avast on the exchange server is blocking it.
Blocking? It shouldn't block anything... avast just scans it.
Can you check if the avast services or whatever it is called in a server could connect, scans mail and deliver it to the workstation?
Ports, firewall?
The workstation that gets the mail does not have Avast installed on it could this be a problem ?
It shouldn't be a problem if the mail is scanned in the server it should, as far I understand, be delivered clean to the workstation