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Exchange Online
A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
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Based on my research, it seems that you are reaching the sending limit to external recipients when using the onmicrosoft domain within a 24-hour window.
For your reference, you may find more details in this article: Email nondelivery reports (NDRs) and SMTP errors in Exchange Online - Exchange | Microsoft Learn
As recommended, it would be advisable to consider using your custom domain for production and commercial purposes. This approach can help ensure a more consistent and reliable experience when sending emails externally.
For more details on setting up a custom domain, you may refer to: Add a Custom Domain to Microsoft 365 - Microsoft 365 admin | Microsoft Learn
I hope this brings helpful insight.
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