The format / protocol do impact and limit what the client can do. In a funny way, thanks to MIME, you can technically do whatever you want with emails, but that will be a poorly organized and structured whatever. In many way, emails are like HTML : A way to present information, not to structure and/or organize it. And therefore, there is only so much the client can figure out about the email. Thanks to Thread-id and the recipient list, you can group emails more easily and more relevantly, but that is about it.