Dear Zoho Support,

We have started receiving delivery delay notifications across all our Zoho Desk departments.

Error details:

• Error: 451 4.7.500 Server busy
• Action: Delayed
• Receiving system: Microsoft 365 / Exchange Online
• Affected Zoho sending IPs identified so far:
  • 165.173.180.51
  • 165.173.182.51

The issue is not limited to one department, recipient, or domain. Some outgoing emails are delivered after a delay, while others remain pending. Our Microsoft 365 administrator is also checking the issue from their side.

Could you please urgently check:

• Whether these sending IPs are currently being throttled by Microsoft
• Whether there is a reputation or deliverability issue affecting Zoho’s sending pool
• Whether there has been any recent change to our outbound sending IPs
• Whether our account can be moved to a healthy sending pool until the issue is resolved

This is currently affecting our daily operations, as our teams cannot confirm whether their replies have reached our partners.

Please let us know if you require the complete email headers, screenshots, or additional examples.

Best regards,
Bora