To try to clarify here (I know Tech and David already know the answer).
The Internet Mail provider (for Outlook Express and all other regular email clients - all of them totally unknown and uncared about to avast) works by intercepting the standard (set in concrete) POP and SMTP email access methods at the network connection level but only for unsecured email connections. (avast and every other AV program cannot scan secured email streams - that is the whole point of them being secured).
The Outlook/Exchange provider for MS Outlook is far superior to the Internet Mail provider. Microsoft has defined an interface for MS Outlook that the Outlook Exchange provider is designed to work with. In this case MS Outlook handles all of the network connections (whether secured or unsecured - makes no difference). However, via the interface, MS Outlook itself presents each message to avast to be scanned and so every message, irrespective of the connection type, gets scanned by avast.
The only other comparable feature is the avast plugin for The Bat mail client.
