Outlook 2007 – 2013 ignores margin and padding CSS styling when placed within an image.
HTML Email Knowledge Base
If your email exceeds 102KB in file size, Gmail will display the first 102KB along with the the message “[Message clipped] View entire message”.
The desktop and mobile version(s) of Gmail automatically convert phone numbers and email addresses to links.
You can target Yahoo! Mail by using the @media yahoo media query and adding your styles.
You can target AOL Mail by including the CSS class .aolReplacedBody and adding your styles.