Send & Receive Issues

How to Fix Exchange Online Error 550 5.0.350 (DNS Blacklist)

Olivia Miller Oct 9, 2026 868 views

Question details

Outgoing emails are bouncing back due to the sending IP address being listed on a DNS blacklist, despite proper domain authentication.

How to Fix Exchange Online Error 550 5.0.350 Due to DNS Blacklisting

Product: Exchange Online
Device & OS: not provided
Scenario: Sending emails to external domains through Microsoft Exchange Online.
Observed behavior: Emails are rejected with error code 550 5.0.350 because the recipient server's IP reputation rules blocked the Microsoft sending IP, even when SPF, DKIM, and DMARC checks pass.

Before you start

Verify if the bounce-back error occurs when emailing a single specific recipient domain or if it affects multiple different external domains.

Solution 1Recommended

Run a Message Trace and Contact Support

Since the blocked IP belongs to Microsoft's shared Exchange infrastructure, an administrator must collect trace logs and request a delisting via Microsoft Support.

Exchange Online uses shared IP pools for outgoing mail. If a spammer abuses one of these IPs, it may get temporarily blacklisted by third-party services, affecting legitimate users like you.

Because you do not own the IP address, you cannot request the delisting yourself. Microsoft's backend team must handle this with the blacklist provider.

Determine the Scope

Review your recent bounced emails to see if the 550 5.0.350 error is happening with just one recipient domain or multiple external domains.

Run a Message Trace

Log into the Exchange Admin Center (EAC) as an administrator, navigate to Mail flow > Message trace, and run a trace for the affected outgoing messages.

Collect the Headers

Open the failed message details in the trace results and copy the complete Non-Delivery Report (NDR) along with the full message headers.

Open a Support Ticket

Go to the Microsoft 365 Admin Center, navigate to Support > New service request. Provide the collected headers and message trace logs, and ask the support team to investigate and escalate the IP blacklisting issue.

Solution 2

Verify Domain Authentication Records

Ensure your domain's SPF, DKIM, and DMARC records are correctly configured to rule out local authentication failures.

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Frequently Asked Questions

What does error 550 5.0.350 mean in Exchange Online?

This error means your outgoing email was rejected by the recipient's mail server because the sending IP address (which is part of Microsoft's shared Exchange Online network) is currently listed on a third-party DNS blacklist.

Can I remove the Exchange Online IP from the blacklist myself?

Generally, no. Since the IP addresses belong to Microsoft's shared infrastructure, you must contact Microsoft Support. Their engineering team will work with the third-party blacklist provider to request the delisting.

Why are my emails blocked even though my SPF and DKIM are correct?

SPF, DKIM, and DMARC verify your domain's identity and ensure the email was not tampered with. However, recipient servers also evaluate IP reputation. If a spammer previously abused the shared Microsoft IP address handling your email, that IP might get blacklisted, causing your legitimate emails to bounce regardless of your domain's authentication.

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