Outlook 2007 – 2013 ignores margin and padding CSS styling when placed within an image.
HTML Email Knowledge Base
Gmail clips emails exceeding a file size of 102KB
If your email exceeds 102KB in file size, Gmail will display the first 102KB along with the the message “[Message clipped] View entire message”.
Prevent Gmail from automatically converting phone numbers and email addresses to links
The desktop and mobile version(s) of Gmail automatically convert phone numbers and email addresses to links.
Target Yahoo! Mail using CSS Media Query
You can target Yahoo! Mail by using the @media yahoo media query and adding your styles.
CSS Media Queries for targeting Apple iPad
Using CSS media queries to target Apple iPad in both portrait and landscape view.