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Deliverability Letter

451 4.7.500 (S77719)

451 4.7.500 Server Busy (S77719)

Server Busy

The Microsoft receiving server is overloaded and cannot accept more connections.

Microsoft throttles inbound traffic when a datacenter is under load. The block usually clears within minutes if you slow down.

Severity

WARNING

Provider

Microsoft 365 / Outlook

Reference

S77719

Last Reviewed

2025-06-01

How to Fix 451 4.7.500 (S77719)

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

  • ✓Implement adaptive throttling for Outlook domains when this error appears.
  • ✓Distribute mail across time zones rather than one global burst.
  • ✓Monitor connection and message concurrency per tenant.
  • ✓Alert operations teams when S77719 rates spike beyond normal baselines.

Helpful Resources

More Microsoft 365 / Outlook SMTP Codes

Keep these related troubleshooting guides handy while addressing 451 4.7.500 (S77719).

421 4.7.0 – 421 4.7.0 Temporary System Problem
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451 4.4.4 (ATTR5) – 451 4.4.4 ATTR5 Temporary Server Error
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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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