Reset OS X settings

You can use the following one-liner to reset settings.

killall Mail &> /dev/null; mv ~/Library/Containers/ ~/Desktop && open -a Mail

Running the above in Terminal will kill the Mail process, then regardless of whether that succeeded or not, it will move the preferences property list from its location in the container to your Desktop, then if this succeeds it will open

To restore the settings from the backup, move from your Desktop back to the original location: