delivery failed - Your message wasn't delivered because the recipient's email provider rejected it.

Hi @Graham Morris

Thank you for posting your question in the Microsoft Q&A forum. 

Based on the information you have provided, looks like you cannot send emails with attachments.

I am so sorry about the inconvenience you are facing. We have received some reports about this known issue.

Remote server returned 550 5.7.520 - Microsoft Q&A

how do you fix a 550.5.7.520 code. message blocked identified as spam AS(4810) - Microsoft Q&A

To resolve the root cause of this issue, you need to contact MS support team, they will collect logs and data and troubleshoot from the backend.

You can raise ticket via Outlook or in Admin Center(For business account)

Meanwhile, you can try this work around in the picture below.

Or if you are sending with PDF, you can convert PDF file to PNG. It will be sent normally.

Since this bug doesn't affect mobile device, you can draft an email on your computer, then send it with your phone. It will be delivered successfully.

Once again, I apologize for the inconvenience this might cause you. I totally understand how frustrating and time consuming this would be for you.

We will try our best to resolve this issue to enhance customers' experience.

Thank you for your understanding.

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