Hotmail connected to Outlook - all messages are marked as spam but only from Outlook

I have an issue where by a colleague has a hotmail.com account, which has been upgraded to a premium one recently.

This is connected to Outlook 365, alongside his corporate Office 365 email account. The latter sends without issue.

When any emails are sent from the Hotmail account, they are immediately returned with:

Remote Server returned '550 5.7.520 Message blocked because it contains content identified as spam. AS(4567)

If we login to Hotmail.com and send a message, there is no error.

I've removed the account and re-added to Outlook with the same outcome. As this is Hotmail mail servers, sending to Office 365 servers, it doesn't make any sense (to me!).

Any ideas appreciated.

Here's a redacted MTA message - all from envelope is @hotmail.com, with the same return path. As you can see, the same *.mail.protection.outlook.com server is rejecting the message from a protection.outlook.com server:

Yesterday, most people reported the same problem. This was due to Microsoft's recent update in its Anti-spam rule. After this issue, Microsoft reverted the update.

For more info: Outloook can't send and receive emails- Emails blocked as spam

If you still face the problem, you can use the Office 365 Anti-Spam IP Delist Portal to remove yourself from the Office 365 blocked senders list.