When sending an email to a Hotmail, Outlook.com, or Live.com address, you may receive a bounce-back error indicating the message was rejected by Microsoft's mail servers.
Symptoms
You receive a bounce-back (NDR) with an error message similar to:
550 5.7.1 Unfortunately, messages from [xxx.xxx.xxx.xxx] 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.
Key indicators:
- Emails to @hotmail.com, @outlook.com, @live.com, and @msn.com addresses bounce
- Emails to other providers (Gmail, Yahoo, etc.) work normally
- The error references your server's IP address and code S3150
- Other email accounts on the same server may also be affected
Causes
This error occurs when Microsoft adds the shared hosting server's IP address to their block list. Common reasons include:
- Spam from another user — On shared hosting, multiple accounts share the same outgoing mail IP. If one account sends spam (intentionally or due to compromise), Microsoft may block the entire IP
- Compromised email account — A hacked email account on the server was used to send bulk spam to Microsoft addresses
- Bulk email from website — A website on the server sent high volumes of email (e.g., marketing blasts, form spam) that triggered Microsoft's spam detection
- Poor sender reputation — The IP accumulated enough negative signals (spam reports, bounces) to be flagged
Important: This is a server-level issue, not an issue with your individual email account or domain. Your email configuration is likely correct.
Solution
Step 1: Submit a Support Ticket
This issue cannot be resolved by you directly — it requires Cynet to submit a delisting request to Microsoft.
Contact Cynet support with the following details:
| Information | Details |
|---|---|
| Your domain | The domain you're sending from |
| Recipient address | The Hotmail/Outlook address you were sending to |
| Bounce-back message | Copy and paste the full error message |
| Server IP | The IP address shown in the error (xxx.xxx.xxx.xxx) |
How to Submit a Ticket
- Support Ticket: Log in to manage.cynet.com.my → Open a Support Ticket
- Email: Send the details to our support team
Step 2: We Handle the Delisting
Once we receive your ticket, Cynet will:
- Identify the source of the spam that caused the block
- Stop the spam by suspending the compromised account or blocking the abusive source
- Submit a delisting request to Microsoft via their Sender Support portal
- Monitor the request until the IP is removed from the block list
Step 3: Wait for Delisting
Microsoft typically processes delisting requests within 24–48 hours. During this time:
- Emails to Microsoft addresses will continue to bounce
- Emails to all other providers (Gmail, Yahoo, etc.) are unaffected
- You do not need to change any settings on your end
Step 4: Test
Once Cynet confirms the delisting is complete, send a test email to the affected Hotmail/Outlook address to verify delivery.
What to Do While Waiting
Temporary Workarounds
- Use an alternative email — If urgent, send from a Gmail, Yahoo, or other free email account temporarily
- Inform your recipient — Let them know via phone or messaging that emails are temporarily delayed due to a server issue
- Retry later — Some queued emails may auto-retry and deliver once the block is lifted
Check Your Own Account Security
While this issue is likely caused by another user on the shared server, it's good practice to ensure your own account isn't contributing:
- Change your email passwords — Use strong, unique passwords for all email accounts
- Check for compromised accounts — Log in to webmail and review your Sent folder for emails you didn't send
- Review forwarders and autoresponders — cPanel → Forwarders / Autoresponders — remove anything suspicious
- Secure your website forms — Ensure all contact forms have CAPTCHA to prevent spam bots
Prevention
While you cannot fully prevent other users on a shared server from causing blocks, you can protect your own accounts:
- Use strong passwords for all email accounts to prevent compromise
- Don't send bulk email from hosting — Use dedicated email marketing services (Mailchimp, Brevo, etc.) for newsletters
- Keep your website secure — Update WordPress, plugins, and themes to prevent exploitation
- Add CAPTCHA to forms — Prevent spam bots from abusing your website's email functionality
- Monitor delivery — If you notice bounce-backs increasing, report to Cynet early
Frequently Asked Questions
Can I delist the IP myself?
No. Microsoft's delisting process requires the IP owner (Cynet, as the hosting provider) to submit the request. Individual users cannot delist server IPs.
Will this happen again?
On shared hosting, it is possible for the IP to be re-listed if another account is compromised in the future. Cynet actively monitors for spam and takes action to prevent repeat incidents. If you require guaranteed email deliverability, consider using a dedicated email service or upgrading to a VPS with a dedicated IP.
Does this affect emails to Gmail or Yahoo?
No. This block is specific to Microsoft's mail servers (Hotmail, Outlook, Live, MSN). Other providers maintain separate block lists.
How do I know if the IP is still blocked?
You can check the IP status on Microsoft's Smart Network Data Services (SNDS) portal, or simply try sending a test email to an Outlook/Hotmail address.