You may see the following error when trying to send an email from MYOB Acumatica: 
This error occurs when the client secret used by your Microsoft Entra ID application has expired. A client secret is a credential that MYOB Acumatica uses to authenticate with Microsoft and send emails on your behalf. Once the secret expires, Microsoft Entra ID can no longer authenticate the application, so emails cannot be sent.
To resolve this error
Visit the Azure portal to create new keys for your app: Register an application in Microsoft Entra ID. You can also refer to our guide, Setting up system email accounts or Exchange integration, for information about Microsoft Entra authentication in MYOB Acumatica.
Creating and updating client secrets requires access to your organisation's Microsoft Entra ID/Azure environment. If you don't have the required access or aren't familiar with managing app registrations, contact your IT administrator