Non-delivery report 550 5.1.8 Access denied, bad outbound sender - Signature 365

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

Note

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.

Warning

Before removing a blocked user, it is recommended you verify their account or computer has not been compromised and is not being used to send unsolicited emails.For more information see How to determine whether your Microsoft 365 account has been compromised and How to fix a compromised Microsoft 365 account.

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 Exchange Online Protection (EOP).

Exclude Recipient Domains From Signature Injection

Exclude messages sent to one or more recipient domains from server-side injection by creating a mail flow rule in Exchange Online.

Non-delivery report 550 5.4.1 Recipient address rejected: Access denied

Resolve 550 5.4.1 recipient address rejected errors, especially for public folders routed through Signature 365.