The Exchange server is busy and cannot send the message right now. Please try again later.

When sending emails you receive the following 504 response:

We are seeing elevated 504's since March 27 2023 across all regions for all customers, there are no documented changed from Microsoft.

Cause:

This means the Exchange server is busy and cannot handle the request at the given time. This typically happens when the providers' servers are overloaded with incoming requests.

Nylas does not send emails but hands them off to the user’s SMTP server for delivery. This error is raised directly by Microsoft and you should reach out to the email administrator to have a better understanding of the cause and request a reason for the increase.

Resolution

• In a bid to reduce the traffic to Office365 we suggest:
  • Ensure you adhere to the Provider limits 
  • Reducing your retry mechanism, there must be an exponential backoff mechanism that halts after a period, see example, so that you do not overload the provider.
    • We are seeing customers retying 8k times on a 504, this exasperates the issue.
  • We suggest sending emails with a 30 second interval.
• To understand why there has been such an increase, the Application Owners will need to reach out to their customers Mail Administrators who in turn will need to ask Microsoft the reason for this behaviour.

Nylas support can not aid you in resolving this behaviour.

Resources:

Improving Email deliverability

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EWS Throttling in Exchange