Email Delivery Issues with Outlook, Hotmail, and Live Addresses (Microsoft Domains)

Overview

You may notice that some emails sent to @hotmail.com, @outlook.com, or @live.com addresses via MSG91 Email Service fail with the following error:

“Unfortunately, messages from [10X.XXX.XXX.134] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).”

Why This Happens

As per the latest update from our internal team, this delivery issue is specifically related to Microsoft domains (Outlook, Hotmail, Live) and is influenced by Microsoft’s internal filtering and block-listing policies.

Since these policies are controlled entirely by Microsoft, they are beyond MSG91’s direct control. At this time, we are unable to provide a definitive timeline for complete resolution.

Important Points to Note

• The limitation is at the Microsoft provider level.

• IP rotation or replacement may not guarantee a permanent resolution under current conditions.

• The failure that you notice is after retrying email multiple times by rotation using different IP range

• Our team is continuously monitoring the situation and evaluating infrastructure-level improvements wherever feasible.

What You Need to Do

No action is required from your end at this time.

Once Microsoft’s filtering conditions improve or a viable infrastructure-level solution is implemented, email delivery to Outlook, Hotmail, and Live addresses will automatically stabilize.
You can either capture alternate email address of your user and send email to alternate email if the delivery failed to primary email address of the client.

You can implement a fallback mechanism in which if the email delivery fails you may consider reaching out to your users on a different channel (SMS, WhatsApp, RCS, Voice etc)