As far as i know, those are the files you need to zap. (I'm assuming you meant ~/Library/Preferences/com.apple.mail.plist)
Q: When you say you're deleting these files, just what do you mean? Hopefully you're either using the CLI rm, or are putting them in the trash and emptying the trash (and making sure it emptied completely). I also assume that Mail.app is not running while you're doing this.