To send an email with 10 personalized fields from multiple data extensions to an audience of over one million subscribers, Northern Trail Outfitters should create and send to a sendable data extension containing those needed fields using a query activity. A query activity can join and filter data from multiple data extensions and populate a sendable data extension with the required fields for personalization. This will improve the email send performance and avoid timeouts or errors. Reference: https://help.salesforce.com/s/articleView?id=sf.mc_as_query_activity.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.mc_es_send_email_using_a_data_extension.htm&type=5