Hi AriaET,
Welcome to the Microsoft Community.
I understand your reference to the sender receiving a postmaster bounce letter.
This should be some sort of rule or forwarding feature set on your mailbox that is unknown to you. Please refer to the following steps to check.
- Login your Hotmail/Outlook account to Outlook.com.
2.Click the gear-shaped settings icon on the top right of the page. In the Settings screen, under the Mail tab.
Look under the ‘Rule’ ‘Forwarding and IMAP’ option to see if there is some rule or forwarding feature that is causing this issue. Please close or delete them.
As shown in the picture below (Note: the content of the interface is in English)


I hope the above information is helpful to you!
Best Regards,
Jimmy Wang |Microsoft Community Support Specialist