The Mail Shield works/protects email clients, conventional email programs such as Thunderbird (which I use).
However, MS Outlook appears to work slightly differently.
As such there used to be an avast Plug-in for MS Outlook, I don't know if this is still the case (as I don't use it).
Looking at the Mail Shield settings, there is mention of MS Outlook and that it scans IMAP4 traffic, I don't know if your version of MS Outlook supports IMAP4 or or not. If not then perhaps that is where the issue lies.
Are there any attachments on the emails your are sending/receiving ?
Something that I had to do with Thunderbird was to import the Avast Security Certificate. Not sure if you would need to do that or how you would do this in MS Outlook.