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Perhaps It would be a very good feature to include in future versions...
If you were using MS Outpost it would be able to do that as there is a plug-in, which effectively has the avast email scanner inside Outpost. I believe the same is true for 'The Bat' email client, but the Internet Mail scanner operates independent (outside) the email program and you need a third party interface as Oldman and Tech mention.
It would probably be possible to build a plug-in for other emails and that would mean a massive amount of programming effort and which one would you support, there are always new and modified email clients coming out. With the Internet Mail provider, these new and modified email clients, provided they conform to the email protocols, should be able to use the Internet Mail provider.