Optimal layout tips for phones

Sending test emails to yourself and reading them on your phone, is the best way to experience how a large proportion of your readership will be viewing your email.
Things to consider

Emails look WAY longer on a phone, especially if people have smaller phone screens, or the font sizes set to larger sizes.
Make your main CTA (your call to action) high up in your email, ideally the first link in your email - so people aren't scrolling to get to the button or main link you want them to click on.

Here is an example of what an email looked like when viewing on a phone - all the information is visible as soon as you open the email. This is done by keeping the word count low and the top image height small.