Permanent failure — do not retry
SMTP 550 5.7.1
IP range on Outlook.com block list
Outlook.com error S3140 is a 550 5.7.1 rejection meaning part of your sending network is on Microsoft's consumer block list. Microsoft does not document the S-codes publicly; senders report the text as "Please contact your Internet service provider since part of their network is on our block list (S3140)."
How it appears
550 5.7.1 Unfortunately, messages from [203.0.113.5] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.
What causes it
- The sending IP belongs to a range Microsoft has blocked, often a cloud or residential range.
- Your IP shares a block with other senders who sent spam.
- Mail is going out from a home or office connection instead of a proper mail server.
How to fix it
In order — try the first one first.
- Request delisting through the Outlook.com sender support form linked from Microsoft's postmaster site.
- If the range belongs to your hosting provider or ISP, ask them to deal with it. You may not be able to fix it alone.
- Send through a reputable sending service whose IPs are in good standing with Microsoft.
- Join Microsoft's SNDS and JMRP programmes to watch your IP's standing at Outlook.com.
Common questions
- What does SMTP 550 5.7.1 mean?
- Outlook.com error S3140 is a 550 5.7.1 rejection meaning part of your sending network is on Microsoft's consumer block list. Microsoft does not document the S-codes publicly; senders report the text as "Please contact your Internet service provider since part of their network is on our block list (S3140)."
- Should I retry a 550 5.7.1 error?
- No. 550 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 550 5.7.1?
- Request delisting through the Outlook.com sender support form linked from Microsoft's postmaster site.
Other 550 replies
- 550 5.1.1 — mailbox does not exist
- 550 5.7.1 — message refused by policy
- 550 5.4.1 — recipient address rejected, access denied
- 550 5.2.1 — mailbox disabled, not accepting messages
- 550 — mailbox unavailable
- 550 5.1.0 — address rejected, other address status
- 550 5.1.10 — recipient not found (Microsoft 365)
- 550 5.1.8 — bad outbound sender (Microsoft 365)
- 550 5.4.5 — daily sending limit exceeded (Gmail)
- 550 4.4.7 — message expired in the queue
- 550 5.7.25 — no PTR record for sending IP (Gmail)
- 550 5.7.26 — blocked, sender unauthenticated (Gmail)
- 550 5.7.27 — SPF authentication failed (Gmail)
- 550 5.7.28 — unusual rate of unsolicited mail (Gmail)
- 550 5.7.23 — SPF violation (Microsoft 365)
- 550 5.7.124 — sender not in allowed senders list (Microsoft 365)
- 550 5.7.509 — DMARC failed, policy reject (Microsoft 365)
- 550 5.7.606-649 — banned sending IP (Microsoft)
- 550 5.7.705 — tenant has exceeded threshold (Microsoft 365)
- 550 5.7.708 — traffic not accepted from this IP (Microsoft 365)
- 550 5.7.750 — blocked from sending from unregistered domains (Microsoft 365)
- 550 5.7.515 — authentication required for high-volume senders (Outlook.com)
Last reviewed 2026-09-11