Message Stuck in the Outbox Folder in Outlook New

Dear All:

I am using Outlook new under Windows 10 Enterprise and Microsoft Office 365. As of last Monday, I have a message stuck in my outbox folder in Outlook New. The message should not be there as it was actually sent, and the recipient had actually replied to it. If I switch to the old Outlook, then I do not see this message in the outbox folder. If I try to delete the message in the outbox folder in Outlook New, it gets deleted, but it re-appears as soon as I move out and then back in the outbox folder, or if I restart Outlook. Also, when I delete the message, next to the outbox folder name, I still see the number 1, indicating that there still is a message in the folder. I have tried synchronising, and also emptying the outbox folder, but to no avail. This is a very simple message, with no attachments and with just a few lines of text. Any ideas what might be going on?

Thank you in advance.