Short version: Avast Free is blocking receiving email on a Win 7 computer with Outlook 2007.
Hello.
Honestly, I've seen reports of similar problems since the last year, or even longer.
It just always seemed to be that such shields, like Mail Shield and Behavior Shield, cause more problems than they add to protection.
So, I'd recommend you (if you value your personal time) to completely disable both shields (ideally, removing those components would be even better, since you wouldn't be able to accidentally enable them).
Or at least you can disable the Mail Shield, if you have no other issues for now.
Your most important protection comes from the Web Shield and File Shield.
The former will protect you when you browse around the web in your browser, and the latter will prevent you from running harmful programs on your PC (including them trying to run automatically).
In other words, having a separate Web Shield is kinda overprotection, as you shouldn't be able to get infected even if you do download a bad file or it tries to run automatically.
(Also, if possible, it's safer to switch to a web interface (especially great with gmail service) instead of using an extra application like Outlook.)
Anyway, that all is just my opinion, though based on observing how many people complain about various issues, and personal experience with only ever using two shields Web and File one, and having no virus issues for many years.