The best method I’ve come up with for dealing with Gmail’s mobile apps is to code for mobile first, and then use a combination of techniques to display the desktop version where suitable.
That's interesting, Mark – that worked better than I would've guessed. Outlook rendering is more precise with padding and border though:
Are there any advantages to using outline?
This one's useful for adding Outlook support to various different responsive techniques. A more complete example with great email client support here:
Credits to Josh Hughes at FreeAgent who first tipped me about using borders for padding a couple years ago. And of course Justin Khoo at FreshInbox for the Gmail :hover hack.