&gt; Then, a threaded email client.<p>Just to be sure: you are aware that that is a standard feature of email? There are headers that provide threading information, at least if you use non-crappy user agents.<p>&gt; It would help to have the convention that an email with subject but no text, text but no subject, or text the same as subject is displayed like an instant message.<p>No, that would be a terrible idea, you never ever should redefine the semantics of existing messages. If you want to encode a new type of message, use metadata to mark it as such, that is what those extensibility mechanisms are for. Add a header to indicate that the sender is following some new convention, or add a multipart&#x2F;alternative MIME part with a new MIME type that encodes such messages, with a fallback text&#x2F;plain part for clients that don&#x27;t understand the new convention. But whatever you do, never encode new semantics in a form that already has an established decoding&#x2F;interpretation&#x2F;meaning, and also always do it in a way that provides as much of the same functionality as possible with software that doesn&#x27;t know about your new invention.