If you’re seeing errors similar to “550 5.7.1 S3150”, and bounce-backs like:

550 5.7.1 Unfortunately, messages from [your IP] weren’t sent. Please contact your Internet service provider since part of their network is on our block list (S3150).

…you’re not alone. This is Microsoft’s Protocol Filter Agent rejecting mail because the sending IP is on their blocklist.

In short: Microsoft thinks your mail server is a risk or has a bad reputation, and refuses to deliver your messages to Outlook.com, Hotmail.com, Hotmail.co.uk, Live.com, Live.co.uk or Microsoft 365 recipients.

Getting the 550 5.7.1 S3150 error or similar errors when emailing Hotmail, Live or Outlook emails is frustrating and can cause business issues.

Table of Contents

Common Causes of a Microsoft Blocklist (S3150)

Poor sender reputation or sudden email spikes

Even if your IP isn’t currently on public blacklists, Microsoft monitors volume, bounce rates, spam complaints, forwards and forwarding loops. A sudden surge of outbound mail or high complaint rates can trigger a block.

Missing or misconfigured email authentication

If your server doesn’t have proper SPF, DKIM or DMARC setup—or if your reverse DNS PTR records aren’t aligned—Microsoft gets nervous and may block you outright.

Rate limiting or throttling

Microsoft sometimes throttles or “soft blocks” IP addresses that send too quickly or hit thresholds in short bursts. Even without overt spam behaviour, this can lead to S3150 errors.

Persistent listing in Microsoft’s internal sender blocklists

Unlike public DNS blacklists, Microsoft maintains private lists of IPs or subnets that don’t meet their internal standards. Having clean external reputation doesn’t necessarily mean Microsoft will accept your mail.

Step-by-Step Checklist to Resolve the 550 5.7.1 S3150 Error

1. Audit your DNS and mail server configuration

Confirm reverse DNS (PTR) for your mail server IP resolves to a sensible hostname, and that it matches your mail server’s A or MX records.
Publish a valid SPF record that includes your sending IPs.
Sign outbound mail with DKIM, ensuring the selector and public key are reachable via DNS.
Implement DMARC (at least a policy of p=none while testing) with legitimate SPF or DKIM alignment.

Getting these three working reliably is often enough to prevent Microsoft from rejecting your mail outright.

2. Check your IP reputation and sending patterns

Use services like Microsoft’s Smart Network Data Service (SNDS), GlockApps or other reputation tools to see if your IP has complaints, bounce issues or “spam trap” hits.
Look for unusual spikes in volume or a sudden increase in outgoing mail—that’s a red flag.
Check that bounce handling and unsubscribe practices are clean and that you’re not inadvertently sending bulk mail to stale or malformed lists.

3. Submit a delisting request to Microsoft

Once your server configuration is correct and your reputation is reasonable, use the Microsoft sender information portal to request removal from the blocklist.

Go to the Microsoft Sender Information Form at sender.office.com (or its current equivalent).
Provide the rejected IP address, a contact email address, your domain, your mail server hostname, and any corrective actions you’ve taken (updated SPF, DKIM, DMARC, reverse DNS, reduced volume spikes, better bounce handling, etc.).
Confirm the delist via the email Microsoft sends you, and monitor delivery to Outlook/Hotmail users.

4. Monitor post-delisting performance

Keep a close eye on bounce rates, spam complaints and any new S3150 rejections. Microsoft may re-block aggressive senders or those that revert to bad behaviour.
Consider slowly ramping up sending volume (“warming up” the IP) rather than blasting large volumes all at once.
Maintain good list hygiene: unsubscribe bounce addresses, suppress complaint senders, and avoid forwarding loops or auto-forwards that might look spammy.

5. Temporary workaround options

If the Microsoft block is severely disrupting communications, consider:

Rerouting Outlook/Hotmail mail through an alternate relay or SMTP provider while you work on delisting.
Using a trusted third-party transactional email provider or dedicated mail service to handle outgoing messages to Microsoft recipients.
Splitting heavy sending traffic across multiple clean IPs, with proper warming and authentication.

Long-Term Best Practices for Mail Deliverability

Always ensure outbound mail is authenticated (SPF + DKIM + DMARC), and your DNS records are clean and consistent.
Don’t suddenly spike your email volume—introduce new sending IPs or domains gradually (“warm them up”).
Practice rigorous list hygiene: remove hard bounces, honour unsubscribes, and monitor spam complaints.
Use monitoring tools to keep an eye on reputation and feedback loops (Microsoft SNDS, international blacklists, bounce logs, complaint rates).
If your mail server is self-hosted or lightly managed, consider fallbacks or relays to ensure business-critical emails always get through even during reputation issues.

In summary

If your emails to Hotmail, Outlook or Live recipients are bouncing with a 550 5.7.1 S3150 error, it’s Microsoft telling you your mail server’s IP is currently blocked or throttled. That usually means either authentication is missing, your reputation or sending habits need improvement, or Microsoft’s internal lists have flagged your IP as risky.

Follow the checklist above: fix the DNS & authentication, audit reputation and sending patterns, submit a delisting request, and keep a close eye on things going forward. With the right steps and some patience, your emails should start getting through again reliably.

If you are struggling and need some professional help, then reach out to us and we’ll do our best to get you back on track.

Why Live/Outlook/Hotmail Rejects Emails with “550 5.7.1 S3150”