If your email exceeds 102KB in file size, Gmail will display the first 102KB along with the the message “[Message clipped] View entire message”.
HTML Email Knowledge Base
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.
Using CSS media queries to target Apple iPad in both portrait and landscape view.
The Outlook email clients add a spacing above and below images which have a height smaller than 19 pixels.