Where mxAURA stops and HubSpot starts
This line is worth being precise about, because it explains most of what you will ever wonder about.
HubSpot owns: your contacts and their properties; the actual sending; delivery timing and throttling; unsubscribes, subscription types and compliance; email templates; workflows and enrollment; opens and clicks at the delivery level.
mxAURA owns: what the email says; which message a contact gets next; the pieces it is composed from; the sequence and its rules; hubs and articles; and, for agencies, distribution to client portals.
The handoff: a HubSpot workflow reaches the mxAURA action → mxAURA picks the message → mxAURA writes the resolved content into that contact's properties → mxAURA answers HubSpot → HubSpot's template reads those properties and sends.
Two consequences follow.
A "success" in mxAURA means the content was written, not that mail arrived. For delivery, look at HubSpot's send log for that email.
If HubSpot cannot accept the write, nothing sends and nothing is lost. The dispatch is blocked or retried, the contact stays in the workflow, and you fix the cause.