Is there a report that shows how many emails end up in Junk Mail? How does Marketo track that?
It's not possible to find out how many emails are sitting in people's spam quarantines/folders. The definition of this anti-spam feature is that it operates silently after delivery (without informing the sending server). To Marketo, it looks like a successful delivery, just as it would if you sent mail through your corporate mailserver and it went to the recipient's spam folder.