Hi,
for OUTBOUND message You can re-configure "MASS MESSAGES" in Heuristic - Advanced page of Mail Provider settings. For INBOUND messages there is a bug in avast! which will be patch in the next update. But You can increate amount of "mass messages" to avoid problem with inbound messages (avast! now use this value for inbound and outbound messages - which is bad, only outbound messages should be affected).
Thanks!