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
I think the best solution is using responsive web design techniques. So my suggestion would be a fluid email layout that would adjust based on the size of the cellphone screen.
Here is an example: http://stylecampaign.com/blog/?p=85
Note: Writing markup for email is a whole different beast than the browser. Here are a few guides worth looking at:
http://articles.sitepoint.com/article/code-html-email-newsletters/
http://www.mailchimp.com/resources/guides/email-marketing-field-guide/