Reset OS X Mail.app settings

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

killall Mail &> /dev/null; mv ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist ~/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 Mail.app preferences property list from its location in the com.apple.mail container to your Desktop, then if this succeeds it will open Mail.app.

To restore the settings from the backup, move com.apple.mail.plist from your Desktop back to the original location:

~/Library/Containers/com.apple.mail/Data/Library/Preferences/