Edit:
Oops. Sorry for the duplicate answer! Lars-Erik was posting while I was composing!! His answer above is more detailed.
Sean D
End Edit:
I use the Internet Mail provider in the avast! On-Access Scanner for scanning my mail.
If you are using that provider to implement "silent delete" select the Internet Mail provider, then Customize, then Advanced.
On that tab select "Silent Mode" with general answer NO (Cancel)
I assume the Exchange/Outlook provider will have similar options (I do not use Outlook)
The key is to know that "general answer No (Cancel) is the option to delete and or quarantine the infected file.
Hope this helps,
Sean D