Email troubleshooting guides
Outlook delivery errors, organized by problem
Troubleshoot Outlook delivery failures using the exact code and response text.
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Outlook 550 5.1.1: Recipient not foundOutlook 550 5.1.1 means the destination mailbox address could not be found; verify the address, avoid stale autocomplete data, and check the recipient object.
Outlook 550 5.1.10: Recipient does not existOutlook 550 5.1.10 means Microsoft could not find the recipient; verify the address and the recipient object before sending again.
Outlook 550 5.1.20: Multiple From addresses without SenderFix Outlook 550 5.1.20 by generating a valid Sender header whenever a message contains more than one address in From.
Outlook 550 5.5.0: Mailbox unavailable or invalidOutlook 550 5.5.0 reports an unavailable Outlook.com or Hotmail recipient; verify the address and stop retrying until it exists.
Outlook 550 5.7.501: Spam abuse detectedOutlook 550 5.7.501 means Microsoft banned the sending account after detecting spam activity; contain the account, reset access, and contact support.
Outlook 550 5.7.502: Sender bannedOutlook 550 5.7.502 means Microsoft banned the sending account after spam activity; secure it, correct the traffic, and contact support.
Outlook 550 5.7.503: Sender blockedOutlook 550 5.7.503 means Microsoft blocked a sending account after spam activity; contain the incident and request restoration after remediation.
Outlook 550 5.7.606–649: Sending IP bannedOutlook 550 5.7.606–649 means Microsoft banned the sending IP range; stop bad traffic, restore reputation, and request delisting.
Primary sources: Primary technical reference.