It seems you mixed up horizontal and vertical splitting. It happens a lot, I'm afraid. I wish it didn't happen so often. It's tedious to clear up such basic confusion when there are so many important user interface challenges to discuss.
Also 900px wouldn't be a problem if not for a vertical split. 900px < 1024px. JoelSutherland is assuming that the window is divided, with some kind of a navigation pane taking up part of the width. This is the case in most email clients.
Last I checked, horizontal and vertical splitting was ambiguous, but you sound very sure your way is right. Do you have a reference to support your confidence?
For example, both emacs and the Java JSplitPane call a "horizontal split" a split of the horizonal axis, resulting in two left/right panes. Meanwhile, vim calls the same operation a "vertical split."
Also 900px wouldn't be a problem if not for a vertical split. 900px < 1024px. JoelSutherland is assuming that the window is divided, with some kind of a navigation pane taking up part of the width. This is the case in most email clients.