A short while ago I posted this exact same problem but no solutions were given, hence the repost. Actually, the problem has been adressed all over the web, but a solution is nowhere to be found.
To me this is mindboggling. Many people use Outlook professionally and come across unprofessional because of the lack of support regarding this matter. No matter what proposed solution I try, I still get mails back from clients on a varied amount of devices that show me double spacing (line breaks) in my original mail.
I was wondering, isn't there a piece of Greasemonkey, Tampermonkey code that could be written for it ?