Disable Rich-Text message format

Exchange 2007/2010

  • Open Exchange Management Console
  • Expand the tree to Organization Configuration->Hub Transport
  • Select the ‘Remote Domains’ tab

image 1

  • Double-click the ‘Default’ entry in the centre pane (Domain *), to open its properties
  • In the properties dialogue, select the ‘Message Format’ tab

image 2

  • Under ‘Exchange rich-text format, select the ‘Never use’ radio button.
  • Click OK to save the change

Exchange 2013 see this article