When using a Microsoft Office365 SMTP server to send emails from the instance, you  may notice some emails are not being delivered to the recipients.

Checking the logs we can notice the following message:

Warning 554 5.2.0 STOREDRV.Submission.Exception:SubmissionQuotaExceededException; Failed to process message due to a permanent exception with message The message can't be submitted because the sender's submission quota was exceeded. SubmissionQuotaExceededException: The message can't be submitted because the sender's submission quota was exceeded.

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The issue could be caused by exceeding throttling limit for the SMTP Authentication protocol but other causes are possible as well.

Please contact your mail admin and ultimately Microsoft Support, showing them the error.

You can also refer to the discussion on Microsoft forums: ERROR 554 5.2.0