Hello, @Glenn Maxwell,
Welcome to the Microsoft Q&A platform!
Based on your description, I understand that you got the error message “Mailbox unavailable. The server response was: 5.7.60 SMTP; Client does not have permissions to send as this sender.” when using PowerShell for syntax testing.
According to the error message, it’s indicate that the application is trying to send an email from an address that doesn’t match the logon credentials. For example, if your entered login credentials for User0@contoso.com in your application settings but the application tries to send emails from User1@contoso.com. If your application behaves this way, please use Office 365 SMTP relay because SMTP client submission does not support this scenario.
It is advisable for you to configure Office 365 SMTP relay If your application does need to send email from a different address. For more, please refer to this document: How to set up a multifunction device or application to send emails using Microsoft 365 or Office 365 | Microsoft Learn.
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Thank you for your support and understanding.
Best Wishes,
Alex Zhang