Why Your Emails Bounce From Outlook and Hotmail (550 5.7.1 Explained)

A bounce from an Outlook.com, Hotmail or Live address that reads 550 5.7.1 Unfortunately, messages from [your IP] weren't sent is not about the recipient. The mailbox is probably fine. Microsoft is refusing mail from the server you sent it through, and until that is fixed, every address on their system will bounce the same way.

This post explains how to read the error, why it happens, how to get your IP removed from Microsoft's block list, and how to tell a 5.7.1 bounce apart from a real bad address so you do not delete good contacts.

Reading the error

The full reply usually looks like this:

550 5.7.1 Unfortunately, messages from [198.51.100.7] weren't sent.
Please contact your Internet service provider since part of their
network is on our block list (S3150). You can also refer your provider
to http://mail.live.com/mail/troubleshooting.aspx#errors.

• 550 is a permanent refusal and 5.7.1 means "delivery not authorised": a policy decision, not a missing mailbox.
• The IP in brackets is the server that connected to Microsoft. If you send through a marketing platform, that is their IP; if you run your own server or a VPS, it is yours.
• The S-code narrows the reason. S3150 means the IP, or the network around it, is on Microsoft's block list. S3140 is similar. Codes in the S3100 range are reputation blocks; codes in the S1000 range are usually one-off spam filtering.

A key detail: Microsoft returns this at the MAIL FROM step of the SMTP conversation, before the recipient address is even mentioned. That is why it affects every Outlook address at once.

Why Microsoft blocks an IP

Outlook.com has one of the strictest reputation systems among the big mailbox providers. An IP ends up on its block list for a few reasons:

• Spam complaints or spam-trap hits from mail sent through that IP, by you or by someone before you.
• Neighbourhood reputation. Cheap VPS and hosting ranges are often blocked as a whole, because so much spam comes from them. The wording "part of their network" points to this. A brand-new IP in a bad range starts blocked.
• Missing reverse DNS. If the IP has no PTR record, or the PTR does not match the hostname in your HELO, Microsoft treats the connection as suspicious.
• Sudden volume from an IP with no history.

How to get delisted

1. Fix the basics first. Make sure the IP has a PTR (reverse DNS) record, that the hostname in the PTR resolves back to the same IP, and that your mail server announces itself with that hostname. Publish SPF and DMARC for the sending domain; you can check both with the free SPF checker and DMARC checker. Microsoft often refuses delisting until these are in place.
2. Submit the delist request at sender.office.com. You will get an email to confirm, and a decision typically within a day or two. If the IP is in a blocked range rather than individually listed, the form may say so; in that case the fix is a different IP.
3. Sign up for SNDS and JMRP. Microsoft's Smart Network Data Services shows you the reputation of your IPs as Microsoft sees it, and the Junk Mail Reporting Program sends you a copy of every complaint. Both are free and make the next block easier to catch early.
4. Warm the IP back up. After delisting, send low volumes to engaged Outlook recipients for a week or two before returning to normal. A full guide is in email warmup and sender reputation.

If you send through a marketing platform rather than your own server, you cannot do any of this yourself. Contact their support with the bounce text; the IP belongs to them.

Do not delete these contacts

This is the most expensive mistake with 5.7.1. Because the first digit is 5, many systems treat it as a hard bounce and suppress the address for good. But the mailbox was never checked. If you suppress every Outlook and Hotmail address after a block, you lose a large part of your list for a problem on your side.

Tell the two apart by the enhanced code and the text:

Check the bounce logs in your sending platform and, if it suppressed addresses on a 5.7.1, restore them once the IP is delisted. The general rule is in SMTP error 550 5.1.1 explained: a bounce is only about the address when the code says so.

What this means for email verification

Verification tools connect to Microsoft the same way a sender does, so a verifier whose IP is blocked gets the same 5.7.1 reply for every Outlook address. An honest tool reports those as Unknown, because it learned nothing about the mailbox. A careless one reports them as deliverable or undeliverable, and the whole Outlook segment of your results is wrong.

InboxSure treats any rejection that arrives before the recipient is checked as Unknown rather than guessing. We describe how each big provider answers verification requests in how Gmail, Outlook and Yahoo respond to email verification.