[Custom SMTP Email Server] - Outlook365 Error - 432 4.3.2 Concurrent connections limit exceeded.

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[Custom SMTP Email Server] - Outlook365 Error - 432 4.3.2 Concurrent connections limit exceeded.

Note: As of 2024, if you're using Outlook365 as your SMTP server, you might encounter the following error when sending batch emails:

ERROR: Message failed: 432 4.3.2 Concurrent connections limit exceeded. Visit https://aka.ms/concurrent_sending for more information

This error indicates that too many simultaneous connections have been opened, exceeding the server's limit of three concurrent connections.

In early 2024, Microsoft introduced changes to message store and throttling for concurrent connections. Under the new limit, only three concurrent connections are allowed to send email messages simultaneously. If an application attempts to send more than three messages at once using multiple connections, each connection will receive the following error message:

432 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded

Additional throttling limits for the SMTP Authentication protocol include:

• 30 messages per minute
• Recipient rate limit of 10,000 recipients per day

Exceeding these limits causes:

• Per minute limit: Email delivery delays. Any excess in message submission will be throttled and carried over to the following minutes.
• Per day limit: Error message 554 5.2.0 STOREDRV.Submission.Exception

These changes necessitate adjustments to how batch emails are sent to avoid triggering these errors and ensure smooth email delivery.

Resources: https://learn.microsoft.com/en-us/exchange/troubleshoot/send-emails/smtp-submission-improvements

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