Why is Outlook or Hotmail blocking my emails?
If emails you send are being rejected by Outlook, Hotmail, or Microsoft 365 addresses while other recipients receive them fine, Microsoft is blocking email from the server sending your mail. This guide explains what that means and what happens next.
What the error means
Microsoft keeps a private reputation score for the internet addresses that send email to Outlook, Hotmail, and Microsoft 365. When that reputation drops, Microsoft rejects or filters the mail with a bounce error that includes a code such as S3150 or S3140.
- S3150 means the sending address is on Microsoft's block list.
- S3140 means the sending address was blocked because of spam complaints or poor sending patterns.
These codes relate to the sending server's address, not to your domain or your account specifically. That is why one customer's emails can trigger a block that affects other customers on the same server.
Why it happens
Microsoft's reputation drops when recipients mark mail as spam, when many messages bounce, or when an account on the server was sending spam. It can also happen suddenly when a legitimate account is compromised and used to send spam: Why are my emails going to spam or being blocked?
What we do
When you report a Microsoft block, we:
- Confirm the exact error and which sending address was blocked.
- Check the sending history for any compromised accounts or spam activity.
- Request removal from Microsoft's block list for the affected address.
- Where needed, move outgoing email to a clean sending address and confirm delivery with a real test message.
What you can do
- Ask the recipient to check their junk folder and mark your email as "not spam", and to add your address to their contacts.
- If you send bulk mail, move it to a dedicated bulk service. Repetitive bulk mail is the fastest way to trigger Microsoft blocks: Why are my emails going to spam or being blocked?
- Make sure your account has not been compromised. If you see sent messages you do not recognise, change your email password immediately: Email Accounts - Changing password
What to send us
The fastest way for us to help:
- The exact bounce message, including the error code such as S3150
- The sender address and the recipient address
- When it started, and whether it affects one recipient or everyone
- A screenshot of the bounce if you have one
Microsoft blocks are usually resolved within a few days of the request, and we keep you updated until delivery is confirmed.
