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I'd probably create a smart list with filter of "Email Address is..." and insert the entire list of email addresses. You may have to break this up into two separate filters, since I believe each one only allows about 1800. Then, by clicking on the Leads tab, you'll be able to see only those email addresses that are already in your Marketo instance. Export this, and use it to scrub against your original list. You'll end up with only names that are in your instance, and then you can select Default when you import it. I hope this makes sense. There may be a better way, but this is what comes to mind if you're concerned about newly imported records.