Send & Receive Issues

How to Fix Microsoft 365 NDR 5.7.708 Access Denied Error

Maira Mehtab Sep 22, 2026 868 views

Question details

The user is unable to send emails and is receiving a non-delivery report (NDR) indicating that their IP address has been blocked from sending traffic.

Product: Microsoft 365 Exchange Online
Device & OS: not provided
Scenario: Sending outbound emails shortly after migrating to a new Microsoft 365 account or tenant.
Observed behavior: Outgoing email delivery fails completely, returning a bounce message that reads: '550 5.7.708 Service unavailable. Access denied; traffic not accepted from this IP'.

Before you start

Ensure you have the full bounce-back email (NDR) to identify the specific IP address that was blocked, and verify that you have Global Administrator rights to access the Microsoft Defender portal.

Solution 1Recommended

Delist the Blocked IP Address via Microsoft Defender Portal

Use the Microsoft Defender delist portal to manually remove your sending IP address from the Microsoft 365 blocked senders list.

This error typically occurs when Microsoft's automated spam filters detect unusual traffic, such as a bulk email surge right after a migration, and temporarily ban the IP address to protect the network's reputation.

Identify the Blocked IP

Open the NDR bounce message you received and locate the specific IP address mentioned in the 'traffic not accepted from this IP' section.

Access the Delist Portal

Navigate to the official Microsoft Defender delist portal in your web browser.

Submit the Delisting Request

Enter your email address and the blocked IP address into the portal. Follow the on-screen instructions to verify your email and submit the removal request.

Wait for Replication

Allow up to 24 hours for the delisting process to fully replicate across Microsoft's servers before attempting to send emails again.

Processing Time: While some delisting requests take only a few hours, you should anticipate a wait of up to 24 hours for full system propagation.

Solution 2

Contact Microsoft 365 Support for Mail-Flow Review

If the self-service delisting portal does not resolve the issue, a Global Administrator must escalate the block to Microsoft Support.

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Frequently Asked Questions

What exactly does the NDR 5.7.708 error code mean?

Error 5.7.708 means that Microsoft has temporarily blocked the IP address you are sending emails from. This is a security measure triggered when an IP has a low reputation or when unusual traffic patterns (like sudden bulk sending) are detected.

Why did my IP get blocked right after migrating to Microsoft 365?

During or immediately after a migration, users often send a higher volume of emails than usual to update contacts or test the system. Microsoft's automated security filters frequently flag this sudden spike in outbound traffic from a new tenant as suspicious, resulting in a temporary IP block.

How long does it take to delist my IP address?

Once you submit a request via the Microsoft Defender delist portal, it usually takes up to 24 hours for the unblocking action to replicate across all global Microsoft 365 servers.

Can any user open a support ticket to unblock the IP?

No. While anyone can submit an IP to the public delist portal, formal support requests for deep mail-flow and IP reputation reviews in Microsoft 365 require Global Administrator privileges.

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