[MlMt] keybinding to close message window

[Daniel Mann]

Continuing my quest to unify keyboard shortcuts between FastMail and the 
Gmail/FastMail experience, today I'm looking for a way to close the 
currently open message windows using a custom keybinding. I know that 
keybindings work from the message window, as demonstrated by "r" to 
reply, "a" to reply all, "f" to forward, etc.

I normally view the message list without the message preview, and open 
messages with the Return key for viewing in a new window. While the 
standard Mac OS X keyboard shortcut CMD+W closes the window, I want to 
assign a custom keybinding for this action - namely, I want to use "u" 
to close the message window and return to the message list.

Is there any syntax available for a custom keybinding to close the 
message window and return to the message list?

Thanks,

-Daniel