When sending email with co-existence enabled a non delivery report (NDR) is being returned to the sender.
The tenant exceeded the mail threshold for the daily limit imposed by the vendor.
The error is found by logging into Exchange Online and performing a Message Trace.
Detail: Reason 5188.8.131.525 Access denied, tenant has exceeded threshold.
For this issue the vendor recommends following the article below and reaching out to Microsoft support if not resolved.
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