Dear @Gareth Evans,
Thank you for posting on Microsoft Q&A forum.
First and foremost, we sincerely apologize for the inconvenience you've experienced while using our services.
To assist you more effectively, could you please confirm the type of account you're using? Is it a personal account or a business account?
Additionally, could you let us know which version of Outlook you're using and when the issue first started? Have you tried checking the behavior using the Outlook Web App? Could you please provide us some screenshot about the issue? (please kindly remove any personal information from the screenshots).
If you're using a business account and you are the administrator of the business, we recommend trying a message trace to determine whether the message was successfully delivered to your Inbox. Please follow this step to access the message trace in Exchange Admin Center:
Note: Please note that our initial response doesn't always resolve the issue instantly. However, with your help and more detailed information, we can work to find a solution.
Thank you for your patience and understanding. I am looking forward to your response.