Remote server returned '550 5.1.8 Access denied, bad outbound sender AS(42004)'

The error message 550 5.1.8 Access denied, bad outbound sender indicates that the email account you are using has been blocked from sending emails due to suspected spam activity. This typically occurs when the account has exceeded sending limits or has been compromised.

To resolve this issue, you can take the following steps:

1. Check for Compromise: Ensure that your account has not been compromised. If you suspect it has, follow the necessary steps to secure it.
2. Contact Your Email Administrator: If you are part of an organization, reach out to your email administrator for assistance in lifting the block.
3. Review Sending Limits: Familiarize yourself with the outbound sending limits of your email service to avoid exceeding them in the future.
4. Use the Delist Portal: If you are an external sender, you can request removal from the blocked senders list through the Office 365 Anti-Spam IP Delist Portal at https://sender.office.com.

For more detailed guidance on this issue, you may refer to the relevant Microsoft documentation on handling such errors.

Hi @Al Martinelli / Nantucket Sinks

Thank you for posting your question in Microsoft Q&A. 

As my research, the message “550 5.1.8 Access denied, bad outbound sender AS(42004)” typically appears in Microsoft Exchange Online or Office 365 environments when an email is blocked because the sender address is not authorized to send mail through that tenant. This issue is often related to the account exceeding sending limits in Exchange Online. 

To resolve this, you could request your administrator to remove a user from the Restricted entities page in Microsoft Defender.

Go directly to this link Security & Compliance 

On the Restricted entities page, you’ll see a list of blocked users, domains, and apps. 

Use the search bar to find the user by their email address. 

Select the user account you want to unblock. 

Click Unblock at the top of the page. 

(Restrictions are usually removed within 1 hour.) 

Additionally, ask your administrator to check your account for signs of compromise and apply security measures to prevent future issues. 

You can refer more via:  

NDR error code "550 5.1.8 Access denied, bad outbound sender" - Microsoft Support 

Remove blocked users from the Restricted entities page - Microsoft Defender for Office 365 | Micros… 

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