There's a good reason for it. The email wouldn't be from the "fake" address at all, it's still from SharePoint.
There's no way around it through standard settings if you're going to stick with all the Workflow plumbing that comes with SharePoint Designer but you could override it with custom Workflows built with Visual Studio.
Failing that, you could also look at building custom message receive handlers in Exchange and swap out the "from" address based on some email flags.
Paul Leigh
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Designer of PhoPho & PhoTab, photograph frames that double up as tablet and smartphone stands.
www.jackdawdesigns.com