Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello Bewilder Box,
Thanks for posting on Microsoft Community.
Based on your description, seem you already set up both MX record and TXT record properly but issue only occurring in Microsoft CNAME record, you might could refer Add DNS records to connect your domain - Microsoft 365 admin | Microsoft Docs Add CNAME records to connect other services (Teams, Exchange Online, AAD, MDM) to do check there is any improvement.
Your precious time and kind understanding will be greatly appreciated.
Kind Regards,
Tommy