When I try to send email from our email server to hotmail or outlook.com I get

550 5.7.1 Unfortunately, messages from [216.x.x.x] weren't sent.
Please contact your Internet service provider since part of their network is
on our block list (AS3150). You can also refer your provider to

I read through all the troubleshooting tips, and searched stack exchange and Google, it all kind of makes sense, but here is what I can't figure out.

Why are they saying that we're coming from AS3150? AS3150 is NTT America https://bgp.he.net/AS3150

We've never been with NTT America. Our IP is on our own IPv4 block registered with ARIN. We operate on our own AS number and we peer with 4 backbone peers in Canada where we are located. None of our peers are NTT America or their subsidiaries. And even if it was, we're still our own standalone AS with our own reputation.

It is frustrating that we're getting blocked because of NTT America's poor reputation. We're not even related. How do we get delisted? This is obviously some screw up.

Or putting on my paranoid hat, is this because our Canadian traffic gets funneled through NTT America's outsourced solutions for mass NSA sniffing and this is just a side effect that someone forgot to covered up? Obviously this would be bad, but at this point I'm so frustrated that I don't even care. If someone will get us delisted, I will happily let them sniff it ... they're doing it with permission or not anyway :-)

  • Have you checked your IP address against blacklists? mxtoolbox.com/SuperTool.aspx Sep 16, 2017 at 0:36
  • What is the exact IP? Have you tried going through hotmails' support ordeal?
    – sebix
    Sep 17, 2017 at 8:05

1 Answer 1


There is a misunderstanding. AS does mean Anti Spam, it's a rule number and not an Autonomous System Number.

So you are just running in an ordinary spam protection rule at outlook.

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