You have a Microsoft 365 E5 subscription.
A user attempts to send an email message to an external recipient and receives the following error message: "Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send messages outside of your organization. Contact your mail admin for assistance. Remote Server returned '550 5.1.6 Access denied, bad outbound sender'." You need to ensure that the user send email to external recipients.
What should you do?