Why does Outlook block my email when Gmail delivers fine?

Outlook and Gmail use different filtering systems. Outlook weighs IP reputation more heavily than Gmail (which focuses on domain reputation). If you're on a shared IP with poor reputation at Microsoft, Outlook blocks while Gmail delivers. Also: Microsoft's May 2025 authentication requirements may not be met even if Gmail's are.

How do I get unblocked from Outlook?

Go to sender.office.com, submit a delisting request with your sending IP. Include: what caused the block, what you've fixed, and your sending practices. Microsoft reviews within 24-72 hours. Also register for SNDS to monitor ongoing.

What is error 550 5.7.606?

Your sending IP is on Microsoft's block list due to spam activity. Either your IP sent spam or another sender on your shared IP pool did. Fix: submit delisting at sender.office.com. If on shared IPs, contact your ESP about pool quality.

What is error 550 5.7.708?

Access denied, traffic not accepted from this IP. Usually means your sending domain or IP failed authentication checks. Verify SPF, DKIM, and DMARC are configured correctly for Outlook (include:spf.protection.outlook.com for M365 users).

Do Microsoft's May 2025 requirements affect my Outlook deliverability?

Yes. Since May 2025, Microsoft requires SPF, DKIM, and DMARC for bulk senders to Outlook.com. Non-compliant email is junked initially and rejected eventually. Verify all three protocols pass for your sending domain.

Outlook Rejection Reasons: Complete List With Solutions

Common Outlook Rejection Codes

550 5.7.1 — Message Rejected

Meaning: Outlook rejected the message based on sender policy. Common causes: Poor IP reputation, missing authentication, content triggered filters. Fix:

Check IP reputation in SNDS
Verify SPF, DKIM, DMARC all pass
Check IP against blacklists (MXToolbox)
If persistent: submit at sender.office.com

550 5.7.606 — Access Denied, IP Blocked

Meaning: Your sending IP is on Microsoft's internal block list. Common causes: Spam detected from your IP, shared IP pool contamination. Fix:

Go to sender.office.com
Submit delisting/unblock request
Include your IP address and what you've done to fix the issue
Wait 24-72 hours for review
If on shared IPs: contact your ESP about pool quality

550 5.7.708 — Access Denied, Traffic Not Accepted

Meaning: Domain or IP authentication failure. Common causes: SPF/DKIM/DMARC not configured for your sending domain, or the domain is too new. Fix:

Verify SPF includes your sending service
Verify DKIM is signing with your domain
Verify DMARC is published
If new domain: may need warmup period with Outlook specifically

451 4.7.500 — Server Busy

Meaning: Temporary deferral. Microsoft's servers are throttling your connection. Common causes: Sending too fast, volume spike, or Microsoft is experiencing load. Fix: Reduce sending rate. Your MTA should automatically retry. If persistent, reduce concurrent connections and per-minute volume to Microsoft recipients.

550 5.4.1 — Recipient Not Found

Meaning: The email address doesn't exist at this domain. Fix: Remove from your list. This is a hard bounce.

550 5.7.1 with "sender is unauthenticated"

Meaning: Your email failed DMARC authentication check at Microsoft. Fix: Same as Gmail 550-5.7.26. Check SPF/DKIM alignment. Configure custom domain authentication in your ESP. Detailed fix: Gmail 550-5.7.26 fix (same principles apply to Outlook)

421 RP-001 — Rate Limit Exceeded

Meaning: You've exceeded Microsoft's rate limit for your IP. Fix: Reduce sending speed. Implement throttling. Wait and retry.

Microsoft's Sender Support Portal

sender.office.com is your primary tool for Outlook deliverability issues:

Delisting requests: When your IP is blocked
Junk mail reporting issue: When legitimate email goes to junk
Sender Information Form: Provide details about your sending practices

Tips for successful delisting:

Fix the root cause BEFORE submitting
Be specific: "We identified that our bounce rate exceeded 5% due to an unvalidated import on [date]. We've removed all bounced addresses and implemented real-time validation."
Include what preventive measures you've implemented
Be patient — reviews take 24-72 hours

Microsoft SNDS for Ongoing Monitoring

After fixing acute issues, set up SNDS for ongoing monitoring. SNDS shows:

Per-IP complaint rates
Spam trap hits
Filter results
IP status (green/yellow/red)

Check weekly alongside Google Postmaster Tools for complete deliverability visibility.

Outlook-Specific Deliverability Tips

IP reputation matters more at Microsoft than at Gmail. If on shared IPs, consider dedicated.
Engagement-based sending helps at Outlook too — Microsoft tracks engagement signals.
Microsoft May 2025 requirements mirror Gmail/Yahoo: SPF, DKIM, DMARC mandatory for bulk senders.
Junk folder training: When recipients move your email from Junk to Inbox, it trains Microsoft's filter for future sends.
Consistent volume: Microsoft throttles senders who spike volume suddenly.

Practitioner note: Outlook is the second-most-important mailbox provider to optimize for (after Gmail). The rejection codes are more opaque than Gmail's, and the delisting process is slower. SNDS is essential — it's the only way to see how Microsoft evaluates your IPs before problems become rejections.

Practitioner note: The 550 5.7.606 block is the most frustrating Outlook issue. If you're on shared IPs, you may be blocked because of another sender — and the delisting form doesn't always resolve it quickly. This is a strong argument for dedicated IPs if you send meaningful volume to Outlook recipients.

If Outlook is blocking your email and you need it resolved, schedule a consultation — I trace Outlook-specific rejections and submit targeted delisting requests.

Sources

Microsoft: Sender Support
Microsoft: SNDS
Microsoft: Outlook Sender Requirements

v1.0 · March 2026