Hello Allan, this looks like the client-side message index might be out of sync with the server's message index. Can you try the following?
1. Remove the account configuration from the affected user's mail client.
2. Log in to their account via webmail and navigate to Settings.
3. Locate the Synchronized Devices area, and remove the entry associated with that mail client.
4. Add the account back in to the email client and monitor for further issues.
Let me know how this works out for you. Have a great day!
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com