K-9 Mail no longer working with Outlook.com email addresses

When I send from my outlook.com accounts using K-9 Mail I receive the error below.
Sending using bluemail or Outlook on my PC has no problems.

Delivery has failed to these recipients or groups:

Your message wasn’t delivered because the recipient’s email provider rejected it.

Diagnostic information for administrators:

Generating server: HE1EUR02HT228.mail.protection.outlook.com

Remote Server returned ‘550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)’

I suggest you contact the recipient’s email provider to find out why it rejects the message generated by K-9 Mail.