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Deliverability Letter

451 4.4.4 (ATTR5)

451 4.4.4 ATTR5 Temporary Server Error

Temporary Server Error

Exchange Online experienced a transient error processing the message.

ATTR5 indicates the Microsoft receiving server encountered an internal failure. The message can be retried shortly.

Severity

WARNING

Provider

Microsoft 365 / Outlook

Reference

ATTR5

Last Reviewed

2025-06-01

How to Fix 451 4.4.4 (ATTR5)

Follow these steps to resolve this Microsoft 365 / Outlook SMTP error and restore normal delivery.

  • ✓Retry with exponential backoff rather than immediate reconnects.
  • ✓Track the frequency of ATTR5 events to spot regional instability.
  • ✓Escalate to Microsoft support if the error repeats for the same tenant.
  • ✓Keep backup MX routes ready if the tenant enforces hybrid delivery.

Helpful Resources

More Microsoft 365 / Outlook SMTP Codes

Keep these related troubleshooting guides handy while addressing 451 4.4.4 (ATTR5).

421 4.7.0 – 421 4.7.0 Temporary System Problem
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451 4.7.500 (S77719) – 451 4.7.500 Server Busy (S77719)
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452 4.5.3 (ATTR38) – 452 4.5.3 ATTR38 Mixed Regions
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451 4.4.62 (ATTR35) – 451 4.4.62 ATTR35 Wrong Region
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