These styles are removed by the RadEditor control because they are not valid style names in certain browsers. As a result the ConvertToXhtml filter would remove them since it does not understand their meaning. By stopping the ConvertToXhtml filter you will be able to persist the mso-line-height-rule and -webkit-text-size-adjust styling. Please, check the article about Editor filters for topics about enabling, disabling filters and more - http://docs.telerik.com/devtools/aspnet-ajax/controls/editor/managing-content/content-filters.
Using embedded CSS is not a viable alternative due to the fact that our clients are creating emails using the RadEditor (hence the use of the Microsoft Office/mso styles) and not all email clients will render using embedded CSS, and definitely fewer using