Could anyone offer some advice on a Hard Bounce issue | Community
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PennyH
Level 2
February 13, 2026
Solved

Could anyone offer some advice on a Hard Bounce issue

  • February 13, 2026
  • 1 reply
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Could anyone offer some advice on a Hard Bounce issue we have developed this week.

The error is: 
550 5.7.1 Unfortunately, messages from [xxxxxx] 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.


It is affecting Hotmail.co.uk, Hotmail.com, live.co.uk, outlook.com and msn.com domains.

Prior to this we have been sending to these domains without issue since our instance was set up 2 years ago. We are emailing known in life customers (not prospects).

I raised a Marketo support ticket and they have given a copy and paste answer:

Since this issue depends on deliverability practices/IPreputation/domain reputation and marketo support has limited resources (available in open source) we have limited scope to investigate this further, If the issue persists after whitelisting, I would recommend reaching out to your CSM, who can further engage the DLVR team who are experts for this additional investigation. However, in most cases, whitelisting the IP address resolves the issue successfully.

 

I filled in this, Delist IP - Delist IP, Office 365 Anti-Spam IP Delist Portal, the Delist response was “The IP address is not currently blocked in our system”

They gave a link to request unblocking for Hotmail.com/Outlook.com.

I filled in that form and got a support request number, but no email confirmation and I have no idea how long it will take to get a response.

I also raised and IT ticket internally to get access to: Microsoft’s Smart Network Data Service (SNDS)  Smart Network Data Services to see if this provides more information.

 

Is there anything else I can be doing?

Is it best practice to suppress these email domains until I get a response from Microsoft?

 

Thank you for reading

 

Best answer by SanfordWhiteman

Thanks, got the email.

 

Wouldn’t say your DNS configuration is correct, although at the same time I don’t want to imply that the errors noted below are responsible for Microsoft properties blocking your mail. The block is most likely because you either (a) have sent suspected spam yourselves recently (did you accidentally send a large email as operational, for example?) or (b) are in the same netblock as an IP that’s currently hosting a spammer.

 

The DNS errors are:

  1. Your RFC *22 From: domain, email.business.three.co.uk, has neither an A record nor an MX record, so it can’t receive mail.

    Any domain used in the MAIL FROM (envelope), or From:, Reply-To:, or Sender: (headers) should be able to receive inbound email. (This doesn’t mean you have to read the email — it means a reply can be sent at the technical level.)

    In fairness though, being able to receive @ your RFC *22 From: domain is pretty minor in the grand scheme. In contrast, being able to receive @ your RFC *21 MAIL FROM domain is extremely important. That’s where bounces go, and servers will flat-out block you if you can’t get bounces (called “backscatter”). And indeed you can receive bounces @ your MAIL FROM, which is actually envelope.business.three.co.uk.

    So while I 100% recommend adding an MX for email.business.three.co.uk and ensuring mail gets delivered, that alone isn’t gonna solve your blocklisting problem.
     
  2. You don’t have a DMARC record for email.business.three.co.uk. Is it mandatory to set up DMARC? Nope. But DMARC ensures your email is non-repudiatable. So if you’re having delivery problems already, you want recipients to have the highest trust level that stuff that says it’s from you really is from you.

1 reply

JaceBr
JaceBr
Level 3
February 13, 2026

I’m assuming you’ve already set up correct SPF/DKIM for your ip address, but you can verify this is still accurate and nothing has changed. It’s probably fine, but sometimes the IT department may make a change that could cause an issue. I use MX Toolbox for this kind of thing. It’s free. 

Are you using a shared or dedicated ip address? If it’s dedicated, then I think you’re going down the right path. You can set up a blocklist monitor with alerts so you can be more proactive in the future too. Something like MX toolbox will do that for you too.

If you’ve using a shared ip address with Adobe then, unfortunately, this might not have anything to do with you as several Adobe customers are using the same ip address and it very well might be their issue. If that’s the case then you can purchase a dedicated ip address with Adobe and start using that. It’s starting over with a sender reputation, and you’ll need to re-set up all your ip validation and may need to roll-back volume as you re-warm a new ip address. But in the long run it may be worth it. 

    P
    PennyHAuthor
    Level 2
    February 13, 2026

    Thank you, we do have a dedicated IP. And we do have the SPF/DKIM set up correctly. I have emailed our IT team just incase something has changed. I will try MX Toolbox, thanks for the recommendation. 

    SanfordWhiteman
    SanfordWhiteman
    Level 10
    February 13, 2026

    What is the IP, and what is your branded envelope sender domain?

     

    In any case, if you're getting a dedicated IP legitimately blocklisted that's gonna be really tough to remedy. Marketo still (of course) pulls IPs from the same netblock so contagion is possible.

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