I’ve done a more recent test at Litmus, with data URIs for inline <img> elements and css background images.
These desktop clients do show data URIs:
- Apple Mail 5
- Apple Mail 6
- Lotus Notes 8
- Outlook 2003
- Thunderbird 3.0
- Thunderbird latest
These mobile clients do show data URIs:
- Android 2.3
- Android 4.0
- BlackBerry 5 OS
- iPad
- iPhone 3GS
- iPhone 4S
- iPhone 5
None of the webmail clients showed data URIs.
These desktop clients don’t:
- Lotus Notes 6.5
- Lotus Notes 7
- Lotus Notes 8.5
- Outlook 2000
- Outlook 2002/XP
- Outlook 2007
- Outlook 2010
- Outlook 2011
- Outlook 2013
These mobile clients don’t:
- Gmail (Android)
- Outlook.com (Android)
- Yahoo (Android)
- BlackBerry 4 OS
- Symbian
- Windows Phone 7.5