Request to Remove IP Block – Error 550 5.7.1 (S3150)

Submit the S3150 case through the Outlook.com consumer sender-support form, using the outbound IP and complete bounce message; the Microsoft 365 delist portal is intended primarily for mail rejected by Microsoft 365 tenants, not Hotmail/Outlook.com consumer recipients.^1^

Submit the mitigation request

1. Open the Outlook.com consumer delisting form.
2. Provide:
   • The public outbound IP shown in the S3150 bounce.
   • A monitored contact address.
   • The affected sender domain.
   • The complete, unedited non-delivery report (NDR), including:
     • 550 5.7.1
     • S3150
     • Date and time, including time zone
     • Outlook.com/Hotmail recipient domain
     • Sending and receiving server details
3. Describe the issue as: “Outlook.com/Hotmail S3150 mitigation request.”
4. State that SPF and DKIM have already been validated; there is no need to repeat those checks before submitting.
5. Retain the case or confirmation number and avoid sending repeated requests while the first is being reviewed.

Outlook.com provides this sender-support route for delivery problems involving consumer addresses such as @outlook.com, @hotmail.com, @live.com, and @msn.com. Submission does not guarantee delivery or delisting.^2^

Confirm who controls the outbound IP

Because the mailbox is hosted through cPanel, determine whether the rejected IP is:

• Dedicated to your organization: the server administrator or hosting provider should submit the form and remediate the server.
• Shared by multiple hosting customers: the hosting provider must handle the request. Another customer’s traffic or the reputation of the wider IP range can affect delivery, and an individual domain owner cannot correct activity from other accounts on that server.

Ask the provider to confirm that the IP in the NDR is the actual SMTP egress IP and request:

• An Outlook.com S3150 mitigation submission.
• Review of outbound queues and logs for compromised accounts or unusual volume.
• Confirmation that reverse DNS/PTR is valid and that the SMTP HELO/EHLO hostname is appropriate.
• Migration to a clean dedicated outbound IP if the shared IP cannot be remediated.

Microsoft may reject senders that fail reverse-DNS lookup. Outlook.com reputation also considers the sending IP, domain, authentication, list quality, complaint rates, and message content—not SPF and DKIM alone.^2^

If the form reports no block or delivery still fails

Use the Outlook.com Sender Support troubleshooting submission linked under “Sender services, tools, and issue submission,” and include:

• The original S3150 NDR.
• The previous submission/case number.
• The outbound IP and ownership status—dedicated or shared.
• UTC timestamps from recent failed test messages.
• Confirmation that failures occur specifically for Outlook.com/Hotmail consumer recipients.
• Relevant SMTP log excerpts, with recipient addresses redacted.

Sender Support also provides SNDS and JMRP services for monitoring Microsoft-visible IP reputation and complaint data.^2^

Do not use [Email] unless the NDR explicitly instructs you to do so or shows the applicable Microsoft 365 block-list error. Likewise, sender.office.com is the documented self-service portal for Microsoft 365 blocked-sender errors; Microsoft’s documentation separately identifies the consumer Outlook.com form.^1^

References

1. External senders - Use the delist portal to remove yourself from the blocked senders list and address 5.7.511 Access denied errors
2. Sender Support in Outlook.com | Microsoft Support