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

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 may be the result of the destination server being overloaded or 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. Flushing the delivery queue via Status > Message Queues > Delivery > Flush All will cause an immediate attempt to redeliver queued messages unless the receiving server has been marked as down, as discussed in this article:

SMTP Code: "451 4.4.1 [internal] connection was not attempted: domain is marked down