I agree that it’s needed but I disagree that it should be necessarily be built as Open Source (unless there is a full-time developer/malevolent dictator driving things – in which case, it could just as easily be built as a commercial app). Singularity of vision is too important in this type of product creation.
“I agree that it’s needed but I disagree that it should be necessarily be built as Open Source”
I agree, particularly if you’re going to build it in Cocoa. There’s nothing wrong with that as an application foundation, of course. Far from it. But it narrows the addressable market for the project, throttles the volume of potential contributors, and ensures that your level of competition is high in Mail.app.
Tough sledding, in other words.