This question is similar to "Standard way to detect mobile browsers in a web application based on the http request" except for mail clients. For instance, if an email message is opened on the built-in iPhone mail client it will display a version of the message specially formatted for the iPhone. If opened on an tablet or desktop it will display as the complete, full-size version of the email. This is similar in principle to web sites that have mobile-friendly versions of the site that load automatically by detecting the user-agent - but for email clients.
So - is it possible to detect the mail client being used to open an email and format the message accordingly? Perhaps a way to detect the screen resolution?