Yay... So basically this is e-mail being reinvented, only this is slightly more complex/unreliable because you're depending on some sort of Messenger bot to be online to deliver the message instead of the recipient's ISP (providing that he's using an ISP e-mail address)...
Seriously, what's so horrible about leaving e-mails for offline contacts? If the problem is waiting for Hotmail to load, then get a pop3 account...