Email troubleshooting guides
Outlook / Microsoft 365 delivery errors, organized by problem
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Outlook / Microsoft 365 451 4.7.650: Temporary sending IP rate limitOutlook / Microsoft 365 451 4.7.650: Temporary sending IP rate limit. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 451 4.7.850: Temporary sending restrictionOutlook / Microsoft 365 451 4.7.850: Temporary sending restriction. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.1.3: Invalid recipient addressOutlook / Microsoft 365 550 5.1.3: Invalid recipient address. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.1.6: Mailbox moved without forwardingOutlook / Microsoft 365 550 5.1.6: Mailbox moved without forwarding. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.1.7: Invalid sender addressOutlook / Microsoft 365 550 5.1.7: Invalid sender address. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.1.8: Invalid sender domainOutlook / Microsoft 365 550 5.1.8: Invalid sender domain. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.2.1: Mailbox disabledOutlook / Microsoft 365 550 5.2.1: Mailbox disabled. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.2.2: Mailbox fullOutlook / Microsoft 365 550 5.2.2: Mailbox full. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.3.4: Message too largeOutlook / Microsoft 365 550 5.3.4: Message too large. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.5.3: Too many recipientsOutlook / Microsoft 365 550 5.5.3: Too many recipients. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.7.513: Recipient tenant blockOutlook / Microsoft 365 550 5.7.513: Recipient tenant block. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.7.703: Tenant Allow or Block List rejectionOutlook / Microsoft 365 550 5.7.703: Tenant Allow or Block List rejection. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.7.705: Suspicious traffic thresholdOutlook / Microsoft 365 550 5.7.705: Suspicious traffic threshold. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.7.708: Traffic not accepted from IPOutlook / Microsoft 365 550 5.7.708: Traffic not accepted from IP. Understand the response, identify ownership, and follow an evidence-based next step.
Outlook / Microsoft 365 550 5.7.800: Sender or EHLO domain blockedOutlook / Microsoft 365 550 5.7.800: Sender or EHLO domain blocked. Understand the response, identify ownership, and follow an evidence-based next step.
Primary sources: Primary technical reference.