How ti fix

Question

Remote server returned '554 5.7.0 < #5.7.520 smtp;550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)>'

Answer 1

Hello. Does this error appear when sending an email to any recipient or just a specific one? You receive this message after sending the email, right? Are you using Outlook on the web or the Outlook app?

Answer 2

The error indicates that Outlook tried to send a message that the receiving system (Microsoft 365) classified as spam and blocked with code 550 5.7.520.

To fix this, the sending environment must be corrected; only an email admin can fully resolve it. Use these steps:

1. Check the sending domain and volume
   • If the message is sent from a custom domain (for example, @contoso.com) through Microsoft 365, ensure that domain is properly configured as an accepted domain and used legitimately.
   • Large volumes of similar messages, bulk mailings, or messages with spam-like content (suspicious links, misleading subjects, or many external recipients) can trigger spam blocking. Reduce bulk sending and remove spam-like content.
2. Verify the domain is registered and validated in Microsoft 365
   • For organizations using Microsoft 365, all domains used to send mail must be added and validated as accepted domains. Messages from unregistered or misconfigured domains are more likely to be blocked.
   • Email admins should follow the guidance to add and validate all sending domains in Microsoft 365 so they are recognized and trusted.
3. Review connectors and mail flow configuration (for admins)
   • If mail is sent via on-premises or third-party systems into Microsoft 365, verify that connectors are correctly configured and not being used to relay spam.
   • Use certificate-based outbound connectors where appropriate, and ensure the certificate’s domain is an accepted and validated domain in Microsoft 365 or Office 365.
   • Check for unusual or recently created connectors that might indicate abuse.
4. Check for compromised accounts
   • If many spam-like messages are being sent from a user mailbox, the account may be compromised.
   • Reset the user’s password, sign out active sessions, and follow the standard compromised-account response steps (including checking for malicious inbox rules and forwarding).
5. After fixing the cause, request block removal
   • Once domain configuration, connectors, and accounts are verified and corrected, the email admin must contact Microsoft Support to request removal of any sending block that remains in place.

For a home Outlook user (non-admin):

• If using an ISP or third-party SMTP server, contact that provider’s support with the full error; they must adjust spam policies or confirm that the account and domain are not flagged.
• Avoid sending bulk or suspicious-looking messages and remove any content that could be interpreted as spam before resending.

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References:

• NDR error codes 550 5.7.703, 550 5.7.705, 550 5.7.708, and 550 5.7.750
• I receive a 550, 553, or relay-prohibited error when sending email messages