Outlook - people not receiving my emails since installing Avast

I have recently downloaded Avast and since then my emails are not being received. It sends OK and nothing in my outbox but some of emails never arrive. I use outlook 2013 please can anyone tell me if its something to do with Avast and how I can solve this please?

webmail, always trouble free :wink:

try turn off avast mail shield, any change?