So, the other day I was at a client and some Outlook installations were sending blank mails. Some of those were always sending blank mails and others only when written in any language other then English (latin characters).
Googling around there were many results that didn't apply, and after some testing, I found it was some add-in that was incompatible.
So if you're having that problem, I'd suggest disabling all Outlook add-ins (even those that you think are OK) and start testing, enabling them one by one again.
The way add-ins are handled, differs through the versions of Outlook. Here's a link with instructions on how to do it in any outlook version: https://www.msoutlook.info/question/88