Hello Keith H,
Thank you for your question.
It could be a temporary issue linked to the Mail application or the windows 10 version, if you are on an old windows version this can contribute to usability issues on the system.
To check your version: Press Windows key + R and type: winver and press Enter
If the version that appears to you is one of these: 1903, 1909, 2004 or 20H2, it means that you are using an old version of windows 10, we recommend that you update urgently.
To update to recent version follow the steps below
https://www.microsoft.com/en-us/software-download/windows10
Step 1: Access the above link and select the Update Now option, an application will be downloaded in which you must install on your computer.
Step 2: when installing you must open the application and the download, verification and installation of the recent version will start.
Step 3: After you will be asked to restart the device to complete the update, after a few minutes you will have the latest version.
If the answer was helpful, please don't forget to vote up or accept as an answer, thanks.
Yours sincerely,
Samuel