I can’t solve your problem. I can only guess: if Juno cannot be downloaded by Outlook Express, is it a pop3/smtp server email? If not, I’m affraid avast won’t scan it. The version 4.5 has a new email detection, automatic, does not require user interaction. But, in fact, it scans pop3/smpt/imap ports and scan your messages.
Anyway, spam killer applications need manually change email account settings. This is by design:
In 4.1, it was like: mail client > avast > spam killer > mail server
In 4.5, it’s like: mail client > spam killer > avast > mail server