Sending, receiving, and organizing email in Outlook.com
Hello,
I've been seeing a sudden increase in reports from other users experiencing this same issue. I suspect this may be an emerging issue on Microsoft's side, although I haven't seen any official confirmation yet.
However, this is an issue on sender's side, not on yours.
Senders need to make sure their SPF, DKIM, and DMARC records are correctly configured for their email address sending domain. I did see a few users reporting that it finally worked because they noticed their DMARC was missing even though they had set up SPF and DKIM. Microsoft didn't require all three records for custom domain senders unless they send 5000 emails a day, but looks like this might have changed yesterday, as it affected many business senders.
If they are already configured correctly on sender's side, but their emails are still being rejected when sending to Outlook.com or Hotmail.com users, they need to submit a report through Microsoft's sender support form. This will help bring the issue to Microsoft's attention. Once again, this is on sender's side, not your side.