Non-delivery report 550 5.1.8 Access denied, bad outbound sender

If you receive the following non-delivery report when you send an email, Microsoft 365 has added the user to the blocked senders list because it detected spam-like activity or the limits of the Microsoft 365 plan were exceeded.

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.

To resolve this issue see Remove blocked users from the Restricted users portal in Microsoft 365.

You can configure Microsoft 365 to send notifications when an outbound spam email is detected, or if a sender is blocked due to sending outbound spam. For more information see Configure outbound spam filtering in EOP.