This article explains the NDR error code "550 5.1.8 Access denied, bad outbound sender," including its causes and possible solutions.
The full Non-Delivery Report (NDR) error message is as follows:
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 email. Contact your email admin for assistance. Remote Server returned 550 5.1.8 Access denied, bad outbound sender.
This NDR occurs when a user attempts to send an email after exceeding the Exchange Online sending limits.
Resolution:
If you're a user, check if your account has been compromised and inform your administrator promptly.
If you're an email administrator, proceed with the following steps.
- Verify whether the affected account has been compromised in Exchange Online.
- Remove the user account block from the Restricted entities page in the Microsoft 365 Defender portal. Once unblocked, all restrictions typically lift within an hour, allowing the user to resume sending emails.
