To fix the problem try one or more of the following: Make sure the SPF records at your domain registrar are properly configured. ...


To fix the problem try one or more of the following:
  1. Make sure the SPF records at your domain registrar are properly configured. Include the following domain name: spf.protection.outlook.com.
  2. If you have a hybrid configuration (some mailboxes in the cloud, some on premises) or if you're an Exchange Online Protection standalone customer, then add the outbound IP address of your on-premises servers to the TXT record.

To learn how see
Customize an SPF record, and Step-by-step instructions for updating SPF(TXT) records at popular domain registrars.