email deliverability issue

Question

hi,

i own a domain hosted by nixihost. out of blue MS has blocked ip address and i can no longer send emails to outlook or hotmail.

reached out to nixihost but they were no help they claim MS needs to lift the block and all they can do is request for it.

this is the error message.

Remote server returned '550 5.7.520 Message blocked because it contains content identified as spam.

Answer 1

Did the error message specifically say that the IP was blocked?

If so, the domain host's response is correct. They cannot self block it. Microsoft has to unblock it. However, Microsoft will not simply unblock it because it was requested, because if the IP is bad, and action is not taken by the service provider to improve the IP reputation score, Microsoft will not just unblock it.

If you are domain owner, you can also try to request it. https://sender.office.com/ However, since you are not the server owner, you will not have access to do anything to improve the IP reputation.

Do you know what the IP is? If so, you can use free tools like MxToolBox to see if it might be also blocked by third party IP monitoring group. In that case, requesting from Microsoft will not help, as you need to request from them first.

Answer 2

Microsoft is blocking messages because they are being classified as spam based on content and/or sending reputation of the IP/domain.

Actions to take:

1. Check the exact NDR
   • Confirm whether the NDR also includes a reference to a blocked/banned IP (for example 5.7.606–649 with a link to https://sender.office.com). If it does, proceed with delisting in step 2.
2. If the NDR mentions a banned IP and sender.office.com
   • Go to https://sender.office.com from a browser.
   • Follow the instructions in the Office 365 Anti-Spam IP Delist Portal to request removal of the source IP from the Microsoft 365 blocked senders list.
   • Complete any required verification (usually via email to an address at the affected domain).
   • After delisting, test sending again to Outlook/Hotmail recipients.
3. Improve IP/domain sending reputation Work with Nixihost (they control the IP and outbound mail server) to ensure:
   • Reverse DNS (PTR) is correctly configured and resolves to a sensible hostname.
   • SPF, DKIM, and DMARC are configured correctly for the domain.
   • The IP is not advertising itself as a non-routable/private address and passes reverse-DNS checks.
   • Any antivirus or gateway security product is not interfering with SMTP (for example, disable “Internet Email Auto Protect/Protection” features and retest).
   • The IP is not brand new or, if it is, that volume and complaint rates are low while reputation builds.
4. Reduce spam-like characteristics in content
   • Avoid spammy subject lines, excessive links, URL shorteners, or misleading “From” names.
   • Ensure “From” and “Reply-To” addresses are valid and match the sending domain.
   • Send to clean, opted-in lists only; remove invalid or bouncing addresses.
5. Monitor and adjust
   • Send a small number of test messages to Outlook.com/Hotmail addresses and verify whether they are delivered, go to Junk, or bounce.
   • If messages start to deliver but land in Junk, continue improving content and reputation; complaint rates and user “Junk” reports directly affect reputation.

Because the IP is owned and operated by Nixihost, only they or the domain owner can:

• Fix DNS/reverse-DNS and authentication records.
• Request delisting for the shared IP if it is on Microsoft’s blocked senders list.

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References:

• External senders - Use the delist portal to remove yourself from the blocked senders list and address 5.7.511 Access denied errors
• External senders - Troubleshoot email sent to Microsoft 365
• Sender Support in Outlook.com
• I receive a 550, 553, or relay-prohibited error when sending email messages