Hi,
I need to know something to narrow down the issue:
1. For the Inbox, what is the account type of the account? Exchange, IMAP or POP?
2. If you move a message to the .pst file, does the issue also occur?
3. Please also let me know the Outlook version that you are using, so that I can also try to reproduce the issue.
The issue may be caused by the corruption in the .pst file, you can try to scan and repair the possible issues in the .pst file, Microsoft provides a tool called Scanpst.exe to do this:
https://support.microsoft.com/en-us/kb/272227
Regards,
Melon Chen
Forum Support