Outlook.com Hotmail emails blocked with error 554 5.7.520 AS(4810)

For a personal Outlook.com/Hotmail account, the most relevant next steps are:

1. Send a simple test message from Outlook.com with plain text only.
   • Since a plain-text message to Gmail was accepted, the block is likely being triggered by the content of the business emails rather than a general account-wide send failure.
   • Remove anything that can look risky to spam filters, such as links, repeated text, or large/embedded content, and test again.
2. Review the message content that is being rejected.
   • The 5.7.520 error is used when a message is identified as spam by mail filtering.
   • If a simple message sends successfully, add content back gradually to identify what triggers the rejection.
3. Check the account for compromise-related changes.
   • Review the Sent folder for unfamiliar messages.
   • Review mail rules and forwarding settings for anything unusual.
   • If anything suspicious is found, change the password and enable MFA.
4. If the issue is specifically with delivery to Outlook.com/Hotmail recipients, use the Outlook.com sender support troubleshooting/contact form.
   • Microsoft provides a sender support path for Outlook.com deliverability issues at:
   • https://sendersupport.olc.protection.outlook.com/pm/Troubleshooting
   • This form is intended for Outlook.com system addresses such as @outlook.com, @hotmail.com, @live.com, and @msn.com.
5. If the rejection is from the recipient side, contact the recipient’s mail provider.
   • When errors indicate the message was blocked as spam by the recipient server, the recipient provider may need to unblock or whitelist the address.
6. If Microsoft support review is needed for Outlook.com account sending restrictions, use the Outlook.com support entry points.
   • In Outlook.com, use Help on the menu bar, enter the issue, and if self-help does not resolve it, go to Still need help? and select Yes.
   • If sign-in or self-help does not work, use the Outlook.com support contact option provided in the support article.

Based on the symptoms described, the strongest evidence points to a content-triggered spam classification rather than a total sending ban, because the plain-text Gmail test was accepted.

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