Dear @Peter Holmes,
Thank you for posting your question in the Microsoft Q&A forum.
As a moderator, I have the same experience as you and other users in this forum. Because this is a user-to-user support forum. Moderators, contributors including external Microsoft employees cannot directly intervene in Microsoft product features or access back-end systems. Our role is limited to providing technical guidance on reported issues, requests, or ideas. However, I will try my best to support you.
First, may I ask what type of account you are using? (Exchange/IMAP/POP)
If possible, please provide your email account details.
Note: Please send this via private message as it contains personal information. To access your private messages, a notification will appear at the top of your post; please click on 'View message.'
However, I would like to clarify one point: Outlook Web is cloud-based, meaning everything is stored in the cloud. If your 365 account uses Exchange (which is standard for work or school), you actually don't need to export anything. Simply signing into your email on the new computer's Outlook app will automatically download and sync all your folders and emails from the server. The export method is only necessary if you are trying to move local folders that aren't synced to the cloud, or if you want a physical backup for peace of mind.
Therefore, please provide your account type and email address so I can double-check this for you.
I look forward to your support and response!