Email in Delivery Queue with "451 4.7.500 Server busy. Please try again later."
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Email in Delivery Queue with "451 4.7.500 Server busy. Please try again later."

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Article ID: 274850

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Updated On:

Products

Messaging Gateway

Issue/Introduction

Messages to a domain hosted in Microsoft Office365 are accumulating in the Messaging Gateway (SMG) delivery queue with the SMTP status 

451 4.7.500 Server busy. Please try again later.

 

 

Cause

This is usually the result of the destination server rate limiting delivery from the Messaging Gateway possibly as part of spam protection policies configured at the destination server.

Resolution

There is no Messaging Gateway resolution to this issue as the decision to accept mail traffic is made by the destination server. Messaging Gateway will continue to attempt to deliver the messages based on its normal delivery retry schedule but flushing the delivery queue via Status > Message Queues > Delivery > Flush All will cause an immediate attempt as redelivery of queued messages.

Additional Information

This appears to have been associated with a specific outage in Office 365 on October 9, 2023 and does not appear to have been repeated.