Hotmail pisses me off. Every time I want to open a new window and reflexively hit the ⌘N keystroke combination, instead of opening a new browser window, it opens a new mail message in Hotmail. I don’t know how Hotmail does it, but I wish it would stop.
I use a PC at the office, and our mail program is Outlook. If I try to open a new window using Crtl+N, it does the same thing. (And Crtl+F is annoying as well.) I think it might be an Outlook thing, rather than a hotmail thing.
In Hotmail (Outlook.com for me now), just go to options (the gear icon in the upper right) -> More mail settings -> Keyboard shortcuts -> Turn off keyboard shortcuts.