A list of all CSS properties supported in Gmail and Inbox.
HTML Email Knowledge Base
Outlook 2007 and 2010 can be a little tricky when it comes to stacking tables side by side or top to bottom, where you may see some undesired gaps between your tables.
Outlook 2007 – 2013 ignores margin and padding CSS styling when placed within an image.
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.