Hello @LeahK48 ,
You'd need to utilize the segmentation tool to set up lists for contacts that meet the criteria you're looking for (List Membership > Not on List).
With such a large amount of contacts, if there's technical limitations in our system then I'd also advise reaching out to our team dedicated for assisting with larger list management - segmentation, imports, etc. For more info on this team, what they can assist with, and how to contact them, please call general support or see this article.
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