The quick answer is: you can't. To send an email to someone, that person must be in the Marketo database. If you have a csv file then you could upload those email addresses to a static list in Marketo, and then send to them. Ensuring you have valid consent of course 🙂 Note that your Marketo database can be a synced-in replica of a different database - for example, Salesforce. If you have the Salesforce / Marketo sync set up, then you are in effect sending to an "external" database. But I don't think that's what you mean.
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