Mail.app error: Could not save to path (but wrong path)
I try to save an inbox message in Mail.app to the Desktop.
Mail.app displays "Error: Could not save to path /Users/pool/Desktop/suit/Pool.txt".
Note that the path it complains about is not the path that I am saving to. I am saving to /Users/pool/Desktop/Pool.txt. Mail.app is inserting an extra "/suit" into the path. Then, of course, it can't save to that path, because that path doesn't exist. It did exist earlier.
This behavior has survived a restart of the machine.
This error never occurred for me in Mountain Lion, but it is occurring now in Mavericks.
MacBook Pro (15-inch Early 2011), OS X Mavericks (10.9.1)