Voting buttons for external recipients - Outlook 2013 to 2010

As long as the message is in RTF or HTML format it should work.  
The admin needs to confirm that Exchange is not set to never send RTF to the internet. In Exchange managment console, organizational config, Hub Transport, Remote Domains tab - Rich text settings are at the bottom of the dialog. The admin can configure a default and different settings for specific recipient domains if needed.

If Exchange is not converting all RTF to HTML, set the contact to always use RTF. See the Where to check section at http://www.slipstick.com/problems/alwaysrtf.asp

Hi Xtermie,

Thanks for this, I've checked on my exchange and I can see its set to determined by individual user settings?

Any further thoughts?

perhaps  this is a problem on the receivers end?
or
check if your outlook has the 'Mail Options', the 'Compose e-mail' format is set to HTML. The addressee's Outlook Contact has the 'Internet format' set to ''Let Outlook decide the best sending format'.