I'm led to believe that Yahoo used an old archaic e-mail system behind the scenes known as qmail - developed by a nutter. The same source tells me that Yahoo have invested heavily in writing their own qmail plugins/enhancements as the original nutter who wrote it couldn't care one iota for improving his own product as he thinks it's perfect the way it is.
I would imagine if Yahoo are migrating away from qmail then there would be a lot of pain in store for everyone.