Managing personal Outlook.com account settings, security, and privacy
Dear charlotte clary,
Thanks for using Microsoft products and posting in the Community.
I understand that your email all forwarded to another address and received many return letters. This is indeed confusing.
Please don't worry. I will do my best to help you solve this problem now. You can try the following suggestions:
These bounce-back notifications typically occur when you send or forward an email to a strange or non-existent address. If you haven’t manually done this, it is highly likely that hackers changed some settings in your account before you implementing protective measures, so please check your email rules and forwarding settings in this situation.
Tip: Please check on the computer, as the relevant settings cannot be viewed on the mobile phone.
Firstly, it is recommended that you check by this link: Rules to see if there are any suspicious rules. You can also go to Gear> Mail > Rules on outlook.live.com and check if there are any forwarding rules set up.
In addition, please click this link to check Forwarding. Sign in the Forwarding and IMAP, then you can see the below display and check if there are any issues with the suspicious forwarding address and if keeping a copy of the forwarded emails is open.
If there is, please disable and delete it, the problem should be resolved.
In addition, we suggest that you take more security measures to protect your account based on the content of this link: How to help keep your Microsoft account safe and secure - Microsoft Support
Hope this help you, please feel free to let me know how it goes. If there is any deviation in my understanding, please forgive me and provide me with more details. I am more than happy to continue to help you.
Best Regards,
Addie.C - MSFT | Microsoft Community Support Specialist