The list of mail accounts in the settings dialog box places a checkbox to
enable/disable each account in turn. Additionally, there is a toggle button
to the right of the list (aren't those something the HIG recommends
against?) that has the exact same functionality.

In light of this duplicate interface elements, I'd propose to remove the
button to the right.

At the same time, I think it would be nice to add reorder buttons ("move
up"/"move down") for selected accounts. This is a request I've seen
elsewhere, but I don't remember seeing a reply as to whether or not this
was something that would be implemented. The column of buttons seems to me
a perfect place to add this functionality.