Microsoft 365 / Outlook 550 5.7.511: banned sender.

Exchange Online Protection banned the sending IP or domain for abuse.

How to fix 550 5.7.511.

Retrying or final
Final
A 5xx reply. The server will not accept this message. Resending it unchanged fails again.
Who is at fault
Sender
Your authentication, reputation, content, or sending pace.
Severity
CriticalSender Block
Permanent550 5.7.511

5.7.511 is Microsoft’s strongest block for IP reputation problems. You must show remediation before Microsoft removes the ban.

Next steps

  1. 1Forward the original NDR to delist@messaging.microsoft.com with full headers.
  2. 2Document corrective actions (list cleaning, compromised account remediation, etc.).
  3. 3Pause traffic until Microsoft confirms the unblock to avoid extended bans.
  4. 4Warm a fallback IP in case the review process takes several days.
Microsoft Delisting GuidanceMicrosoft 365 / Outlook documentationEvery raw SMTP 550 reply

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Full Microsoft 365 / Outlook playbook

Other Microsoft 365 / Outlook fixes

421 4.7.0Temporary system problemRetrying
451 4.4.4 (ATTR5)Temporary server errorRetrying
451 4.7.500 (S77719)Server busy (S77719)Retrying
452 4.5.3 (ATTR38)Mixed regionsRetrying
451 4.4.62 (ATTR35)Wrong regionRetrying
451 4.7.650Sender blocked by EOPRetrying

In the glossary

Bounce (Hard)Hard bounce is when an email cannot be delivered because the address is invalid or blocked. It is a permanent failure and the address should be removed from your list.
Suppression ListA suppression list is the place your email tool keeps addresses that should not receive messages. It is a permanent "do not send" list that helps you avoid emailing people who have opted out or caused issues in the past.
List HygieneList hygiene is the practice of cleaning your email list by removing bad or inactive addresses. It keeps your list focused on people who still want your emails.
Bounce RateBounce rate is the percentage of emails that could not be delivered to recipients. It shows how many addresses are invalid or blocked compared to the total you tried to send.

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More references.

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