"Mailbox info is stale" when sending emails using Microsoft Graph's Mail.Send

Question

Hi,

Some users of a software of mine that sends (WordPress) emails using Microsoft Graph have come back, reporting an issue whereby they receive the error message "Mailbox info is stale" when attempting to send emails using Microsoft Graph. Those customers have all discussed this with Microsoft Support, but the issue has not been resolved. When they attempt to trace the message (in Exchange Online Admin), it appears the message never reached Exchange Online and hence no trace exists. When they sign into the mailbox using the web interface, they have no problem accessing the mailbox and they can send emails from the mailbox in question without any problem.

The issue at hand really seems to exist at the interface between Microsoft Graph and Exchange Online. I came across this thread and it seems that this error also occurs for some users that are using the new Outlook client application.

Is this a known issue and how should a customer proceed to get this issue noted by the right team? Because Microsoft Support appears not to acknowledge this issue.

Thanks!