Delivery has failed to these recipients or groups

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This error is caused by exceeding one of the outbound sending limits as specified in the service limits or in outbound spam policies when you are restricted from sending email.

When this happens, you will not be able to send e-mails to that account, When you try to send email, the message is returned in a non-delivery report (also known as an NDR or bounce message) with the error code 5.1.8 and the following text:

"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."

To solve this issue, please contact your organization administrators. They can remove users from the Restricted users page in the Microsoft 365 Defender or in Exchange Online PowerShell.

To remove a user from the Restricted Users list, please follow these steps:

1. In the Microsoft 365 Defender portal at https://security.microsoft.com, go to Email & collaboration > Review > Restricted users. To go directly to the Restricted users page, use https://security.microsoft.com/restrictedusers.
2. On the Restricted users page, find and select the user that you want to unblock by clicking on the user.
3. Click the Unblock action that appears.
4. In the Unblock user flyout that appears, read the details about the restricted account. You should go through the recommendations to ensure you're taking the proper actions in case the account is compromised. When you're finished, click Next.
5. The next screen has recommendations to help prevent future compromise. Enabling multi-factor authentication (MFA) and resetting the password are a good defense. When you're finished, click Submit.
6. Click Yes to confirm the change.

For more information, please visit Remove blocked users from the Restricted users portal - Office 365 | Microsoft Learn.

Best Regards

I am friendly user. I will try to help you.

If the sender is blocked, kindly try to unblock the sender via using delist portal and check the result after couple of hours.

In a web browser, go to https://sender.office.com.

Article for reference: Remove yourself from the blocked senders list and address 5.7.511 Access denied errors - Office 365 | Microsoft Learn

Hope it can be worked.

Greetings, Tharindu! My name is Arif, and I will assist you today.

1. May I confirm it is only for the one specific user whom you are not able to send emails?
2. If yes the recipient can send you an email without any issue?

• You can ask them to verify their block list or put your email and domain to their whitelist and then try to send the email. According to the error notice, your email before it was sent from the receiver end was suspected of being spam.

Warm Regards,

Arif

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