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This episode is all about the 550 5.1.8 Access denied, bad outbound sender" NDR and provides actionable insights for resolving this error.

Core Issue: The error message "550 5.1.8 Access denied, bad outbound sender" signals that Microsoft 365 has blocked an email account from sending outbound messages. This typically occurs due to suspected spam activity.

Key Takeaways:

• Trigger Points: Microsoft may trigger this error if:
• The account is compromised or exhibits suspicious activity.
• Sending limits (daily, hourly, or recipient-based) are exceeded.
• Outbound email content triggers Microsoft's spam filters.
• Recipients mark the sender's emails as spam.

1. Initial Steps:Security Check: Verify the account hasn't been compromised. Enable multi-factor authentication (MFA) and reset passwords if needed.
2. Unblock User (Admin): Microsoft 365 admins can unblock users directly through the "Restricted Users Page" within the Defender Portal.

• Addressing Spam Concerns:Review Outbound Policies: Check the default "Anti-spam outbound policy" and consider increasing volume limits cautiously. Excessive volume can negatively impact deliverability and reputation.
• Refine EOP Settings: Leverage Exchange Online Protection (EOP) to customize outbound anti-spam policies, including setting up notifications for suspicious emails.
• Best Practices:Adhere to email deliverability best practices to avoid triggering spam filters.
• Consider using a separate domain or subdomain for cold emailing to protect the reputation of your primary domain.

Important Considerations:

• Quote: "Microsoft disables accounts from sending outbound messages when the email account hits an undisclosed threshold. 'We monitor accounts that are sending spam, and when they exceed an undisclosed limit, the account is blocked from sending email. There are different thresholds for individual users and the entire tenant' — Microsoft 365 EOP"
• Impact: Continuous deliverability issues despite implementing the recommended fixes may indicate a need to establish a new domain for cold outreach.

Next Steps:

• Implement the recommended troubleshooting steps based on the specific cause of the error.
• Consult Microsoft's documentation for detailed information on managing outbound bulk emailing and anti-spam policies.
• Consider engaging email deliverability experts for further assistance if issues persist.