HTML: GUI: add a right margin for fancy button formatting
Document the shortcuts of the other MUAs
GUI: add a "smart" replying mode which will just pick the "default action"

It's better to call this a "guess", not a "smart mode", really.
GUI: add shortcuts for the different replies and the message composer
Change the default replying mode to "reply all"

Let's see how popular this is going to be...
Backend functionality for "reply to all but me"
GUI: add the "Reply to All but Me" action and make it preferred over the "Reply to All" thing