I have a regular newsletter which goes out to an opt in list. How do I go about creating an email newsletter that will respond to a user's screen size without knowing their chosen method of reading the newsletter?
seriously speaking, that's how i have my email client set to show email, so make sure you take that into account when considering "responsive design". some emails i receive are graphics only, so that's a failed message without further action on my part.
Use a table and a flexible % based layout. No columns (too easy for it to become cramped) - design using all rows and a layout that allows text to 'flow' down the page as the layout is resized.
Images should be minimal using background colours where possible - flat approach wins out. Except for some % widths on logos so they scale inside the parent table cell.
Avoid nested tables and fixed width images.
Lastly I'd add some inline enhancement styles for properties like max-width / min-width so you limit the 'crushing' effect on mail clients that support more sophisticated css (basically everyone but Outlook).
I use Mailchimp for sending newsletters. I've always used my own custom coded HTML but realised I needed to update it to a responsive layout. I spent a long time thinking about it then decided for now the quickest solution was to use one of Mailchimp's free responsive templates. Seems to work well.
Mailchimp has several RWD templates published on Git. You'll also find other RWD email templates in Git. The net-net is that RWD email templates don't work yet. You'll need to settle on a 320px wide template for the time being. If you're dead-set on RWD you're going to spend a ton of time trying to make it work. I blew 100+ hours before I gave up.
I have a client who uses Constant Contact. They have a 600px wide footer that cannot be changed. So, if you're using CC, forget about anything less than 600px wide. As an FYI - for a few hundred $$ CC will build you a smaller footer (I'm not a fan of CC). In general, all of the ESPs are behind the curve on mobile friendly email. It is so very sad. Let's hope the industry responds to our needs.