Dear mik256
Thanks for choosing Microsoft Community.
Based on your description, I know the scenario you met. The yellow envelope icon in Outlook typically indicates that the email message has not been read yet. However, since you mentioned that opening and reading the email does not make the icon disappear, it could be a bug or a glitch in the software. For this, please be kindly to try the suggestions bellowing and check if these can make any difference.
- Repair Office Application: Repair your office application: You can refer to Repair an Office application (microsoft.com) and then check the result.
- Create a new profile according to Create an Outlook profile - Microsoft Support.
- Use SaRa Tool to fix your outlook issue: You can get more detailed information from About the Microsoft Support and Recovery Assistant.
- Un-install outlook and re-install it.
Hope the suggestions above can help you. If the issue persists, please be kindly to provide me the screenshot about your Outlook version. I will continue to help you based on the information you provide. Your understanding is highly appreciated.
Thanks for your precious time. Have a nice day.
Best Regards,
Katherine