Hi,
Please first make sure you have installed all available updates for your Office 2016, then check whether this issue continues.
Does this issue happen if we try starting Outlook in safe mode? Press Win key + R to open the Run command, type
outlook /safe and press Enter. If this issue doesn't happen in safe mode, then we may try disabling all add-ins to see the result.
Besides, we may also try recreating the Outlook email template via renaming the NormalEmail.dotm file. Then restart Outlook to see the result. By default, the NormalEmail.dotm file is stored in the following location:
drive:\Users\%username%\AppData\Roaming\Microsoft\Templates
Hope this helps.
Regards,
Steve Fan
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