Every now and then we get a Marketo Unknown lead, and even though most information is not available, the email address seems to be. How is Marketo getting this email info? And is there a way of capturing info in other fields as well?
Double click the New Lead activity for the records in question and see what is listed as their Source Type. This will tell you where the leads are originating from and then you can figure out what is passing the email address into Marketo.