Troubleshooting 4 different types of Microsoft DKIM rejections

If your attempts to send email to Microsoft-hosted inboxes are bouncing back intermittently with authentication-related rejections; specifically highlighting that DKIM is failing, when you know you’ve got DKIM authentication correctly implemented: what do you do now? The actual SMTP rejection looks something like this: “550,5.7.515,Access denied, sending domain example.com doesn't meet the…