550 5.1.8 Access denied, bad outbound sender AS(41000001)
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 email admin for assistance.

This error means that Office 365 (Microsoft 365) has added the user to the blocked senders' list because this person's emails have been recognized as spam. This might happen, for example, if the user has sent an email to a large number of recipients, exceeding the limits of the Office 365 plan (you can check the Office 365 sending limits for individual recipients and distribution groups in this Microsoft's article).