It isn't clear to me whether this e-mail system is going to be an e-mail server or an e-mail client. Or both. You say it's going to work on handheld devices, and that suggests to me that it's a client. But then you start asking about Yahoo and Hotmail, which are servers.
I can't imagine running an e-mail server on a handheld device. So I suppose you must be asking about a client, which will connect to a central server somewhere. Am I right so far?