Hi @Peter Volz ,
From your description, in the case of 4k or higher resolution, there is a gap in the background color of the email in RTF format. To further confirm, can this issue be reproduced with low resolution screen?
According to my research, if you have a multi-monitor setup (such as a high DPI laptop connecting to a low-DPI external monitor via a docking station) you may see some awkward display issues when you move Office apps back and forth between the displays. For more information see: Office support for high definition displays.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.