Fully Supported
All the options work properly. Fallback elements show for image backgrounds and web fonts.
Desktop
Apple Mail 8, 9 & 10
Microsoft Outlook 2007, 2010, 2011, 2013, 2016
Windows 10 Mail
Thunderbird 45
Airmail2
Mobile
Gmail App (Android & iOS)
iPhone (all)
iPad (all)
Web
AOL
Comcast
G Suite
Gmail
Inbox by Gmail
Mail.ru
Office 365
Outlook.com
Web.de
Yahoo! Mail

Partial Support For Some Options
Messages display correctly, but some options (with low impact) may not display as expected.
Desktop
Microsoft Outlook 2007, 2010, 2013 - Borders are not fully supported
Mobile
Android 4.4 to 6.0 default email application (this is not Gmail) - Columns don't expand to cover 100% of the available width
Web
Office 365 - Borders are not fully supported
Outlook.com - Borders are not fully supported
Only basic settings works
When sending to these clients, we recommend using text and images in a simple one-column layout.
Desktop
Lotus Notes 7, 8 & 8.5
IBM Notes 9
Outlook 2000, 2002 & 2003