Hi Martin Noel,
Welcome to Microsoft Q&A forum.
To narrow this down, could you please confirm:
- Where are you seeing the “only today’s emails” inbox — Outlook.com in a browser, the Outlook desktop app (new/classic), or the Outlook mobile app?
- When the older emails “disappear,” are they showing in Deleted Items (including Recover items deleted from this folder) or Junk Email?
- In Outlook.com settings, do you see any Mail Rules (especially move/delete/forward/redirect), even if you didn’t create them?
If your account showed unfamiliar successful sign-ins, Microsoft’s “compromised account” steps are also here: How to recover a hacked or compromised Microsoft account.
Once I have the answers above, I can point you to the right next steps for your exact setup.
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