I was inspired to Google after (yet again) getting frustrated by inconsistent user experience in Microsoft Outlook.

http://www.codetwo.com/blog/double-spacing-in-outgoing-messages-in-ms-outlook-2007-2010-2013/

Short version; Outlook's trying to act like a professional word processing package when you use 'HTML' format for email creation, and actively using behavior contrary to pretty much every other MUA.
What's new?

I ordinarily enjoy using Outlook as an MUA but it does have a couple of annoying quirks, this is definately one of them. The content of the above link is likely to be useful.