I am experiencing a persistent issue when attempting to send emails via SMTP using Microsoft Exchange Online in a NodeJS application. The authentication process appears to be successful, but email transmission fails with the following error:

Message failed: 430 4.2.0 STOREDRV; mailbox logon failure; STOREDRV.Submission.Exception:ConnectionFailedTransientException.MapiExceptionLogonFailed; Failed to process message due to a transient exception with message [BeginDiagnosticData]Cannot open mailbox.

The implementation in place worked seamlessly for almost a whole year but recently started raising this error whenever an email is attempted to be sent from our server.

I’ve already checked the application’s credentials and permissions and everything seems in place for this setup. The sender’s email configurations have also not been changed since the first implementation of the system.

I’m using NodeJS with nodemailer and @azure/msal-node package for the credentials management.

Any idea for the root cause of this issue? Looking online hasn’t yielded much information on this error in particular and I’m currently stuck in a dead end, so any ideas on how to further debug the issue would be highly appreciated.

Thank you in advance