Hi @Kelly Rattanee ,
According to the information, the error message is "Remote Server returned '550 5.7.705 Service unavailable. Access denied, tenant has exceeded threshold. " When there are too much spam or bulk mail has been sent from your organization so Microsoft blocks users in your organization from sending outgoing emails.
Here is a link about this error for your reference: "Tenant has exceeded threshold" NDR when mail is sent from your organization
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