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Fix NDR error code "550 5.1.1" through "550 5.1.20" in Exchange Online

Serah Githambo 0 Reputation points
2025-10-24T06:33:03.63+00:00

I have been doing an email outreach but the email keeps getting blocking and getting this error
Fix NDR error code "550 5.1.1" through "550 5.1.20" in Exchange Online
This message couldn't be delivered because the sending email address was not recognized as a valid sender. The most common reason for this error is that the email address is, or was, suspected of sending spam. Contact the organization's email admin for help and give them this error message.

But I have checked spam words, and my email does not have any. Sometimes, I have already reached out to the same people, but when sending the second email I get the same problem.
I have tried unblocking, but after send successful emails it gets blocked again
How do I find a long tern solution to this problem
A high-severity alert has been triggered

⚠ User restricted from sending email

Severity: ● High

Time: 10/22/2025 6:00:00 AM (UTC)

Activity: Potentially compromised user account

User: [PII removed]

Details: User [PII removed] has been restricted from sending messages outside the organization due to potential compromised activity.

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Exchange Online
A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
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  1. Austin-H 5,945 Reputation points Microsoft External Staff Moderator
    2025-10-25T06:33:23.6933333+00:00

    Hi @Serah Githambo
    Please understand that our forum is a public platform, and we will modify your question to cover your organization's domain name in the description. Please note that you should hide this personal or organizational information the next time you post a bug or some information to protect personal data.
    Thank you for posting question to Microsoft Q&A Forum.

    According to this issue, may I know if you are also the Global Administrator of this tenant? Did you try to release this user from Microsoft 365 Security & Compliance Center > Review > Restricted Identity before and this user get blocked again?
    For this user, it's recommended you should perform a password reset and make sure that Multifactor Authentication (MFA) is enabled upon logging into this account.
    Please also check on your Outbound Anti-spam Policy Settings if this has any limits or suspecting factor.
    If you have not reached out to Microsoft Support Team within Microsoft 365 Admin Center before, consider raising ticket to them if this issue persists even after the given suggestion above. They should be able to collect information from your end and they could investigate this issue. As I am forum moderator, I do not have access to Microsoft internal system to check on your side.
    Please let me know if you have any new updates, even from the Microsoft Support Team.

    Note: Follow the steps in our documentation to enable email notifications if you want to receive notification related to this topic.

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