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

Dear Tailored,

I hope you’re having a good day. 

The error "550 5.1.8 Access denied, bad outbound sender AS(42004)" typically indicates that Exchange Online has restricted the mailbox from sending email because the account was identified as a potential spam sender or showed activity that triggered Microsoft's outbound protection systems. This can occur if the account was compromised, exceeded sending limits, or generated suspicious outbound email activity.

As a Microsoft 365 administrator, the first place I would check is the Restricted entities page in Microsoft Defender:

1. Go to Microsoft Defender Portal
2. Navigate to: Email & collaboration > Review > Restricted entities
3. Search for the affected user's email address.
4. If the user appears in the list, review the details and select Unblock after confirming the account is secure. Microsoft notes that restrictions are typically removed within about one hour after a successful unblock.

I would also recommend investigating whether the mailbox may have been compromised before unblocking it. For example:

• Review recent sign-in activity.
• Check for unusual mailbox rules or forwarding rules.
• Review recent outbound message volume and recipients.
• Confirm MFA is enabled and reset credentials if compromise is suspected.

Microsoft's official guidance for this NDR can be found here: NDR error code "550 5.1.8 Access denied, bad outbound sender"

I hope this information helps point you in the right direction.

Warm regards,  

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