@Addiss said
The sender confirms they are being bounced back
This usually means that the emails are being rejected by the Microsoft email server before it ever gets to your Outlook client. Microsoft has a number of checks they do on incoming emails and if it fails, then the sender gets a Non-Delivery Report (NDR).
Please ask the sender to look at the NDR. It should have a section where it says why Microsoft rejected the email as an error code like "5.X.X" e.g. 5.1.1 (bad destination mailbox), 5.7.1 (delivery not authorized), 5.2.2 (mailbox full), etc.