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How to Resolve Microsoft 365 NDR Error 550 5.7.705
Question details
The user needs to resolve an email delivery failure caused by a blocked sender, IP address, or tenant due to suspicious activity.
- Product
- Microsoft 365 Exchange Online
- Device & OS
- not provided
- Scenario
- Attempting to send emails after a tenant compromise or spam outbreak, resulting in the sending address or tenant being blocked.
- Observed behavior
- Emails bounce back with Non-Delivery Report (NDR) Error 550 5.7.705, indicating the sender or tenant is blocked from sending mail.
Ensure you have Global Administrator privileges in your Microsoft 365 tenant, as you will need access to the Exchange admin center and security portals to investigate and resolve this issue.
Investigate and Secure Compromised Accounts
Identify the source of the spam activity and secure the affected accounts to prevent further unauthorized access.
Error 550 5.7.705 is usually triggered when Microsoft detects a high volume of suspicious emails originating from your tenant. Securing the compromised accounts is the critical first step.
Log into the Exchange admin center, navigate to Mail flow > Message trace, and identify which accounts are sending out spam.
Go to the Microsoft 365 admin center, select the compromised users, and force a password reset.
In the Azure Active Directory portal, select the affected user accounts and click 'Revoke sessions' to sign them out of all devices.
Ensure MFA is enforced for all users, especially administrators, to prevent future unauthorized access.
Submit an IP Delist Request
Request Microsoft to remove your IP address from the blocked list once your tenant is secured.
Open a Microsoft 365 Support Case
Contact Microsoft support directly if the issue persists after securing the tenant and requesting a delist.
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Frequently Asked Questions
What triggers the 550 5.7.705 error in Microsoft 365?
This error is typically triggered when Microsoft's automated systems detect suspicious or excessive email activity originating from your tenant. This most commonly occurs if a user account has been compromised and is being used to send spam.
How long does it take to get unblocked after submitting an IP delist request?
Once you submit a request through the Office 365 Anti-Spam IP Delist Portal, it usually takes a few hours for the IP to be removed from the blocklist, although it can occasionally take up to 24 hours.
Can a standard user fix the 5.7.705 error?
No, a standard user cannot resolve this issue. A Global Administrator is required to investigate tenant-level security compromises, remove blocks in the Exchange admin center, and communicate with Microsoft Support.
