Reminder to authenticate outbound email with SPF, DKIM, and DMARC
O365 admins can see full details here Service health – Microsoft 365 admin center
Further advice on successful sending of email via Exchange online (O365) from 21st September 2023 as issued by Microsoft – see link above for full content
Message Authentication, Reporting, and Conformance (DMARC) in Domain Name System (DNS), and you’re not sending a large rate or volume of email that exceeds the sending limits of the service, you’re not impacted and there’s no action required by your organization.
Senders will receive Non-Delivery Reports (NDRs) when sending messages that exceed the sending limits of the service. We recommend that senders don’t send a large rate or volume of email that exceeds the sending limits of the service. This recommendation also includes not sending email to a large list of BCC recipients.
Avoid using addresses in your primary email domain (for example, contoso.com) as senders for bulk email. Doing so can affect the delivery of regular email from senders in the domain. Consider using a custom subdomain exclusively for bulk email. For example, use “m.contoso.com” for marketing email and “t.contoso.com” for transactional email.
We recommend configuring any custom subdomains with email authentication records in DNS (SPF, DKIM, and DMARC). Many email service providers (for example, Gmail, Yahoo!, and Outlook.com) are configured to reject messages that don’t meet email authentication standards.
For additional reading and documentation on best practices, please review the links below.