Hi Xavier -- There a lot of variables on this one. I've done the same thing in the past, but found that a lot of the email addresses listed weren't really my target customer, merely someone watching email from another department, but as long as that's something in your cleansing process, then kudos to you. I definitely would not simply upload them to Marketo without a comms plan in place -- unless you have a pretty solid cold nurture (like I said, lots of variables). Another variable is whether you have double-opt-in or not, but I'm assuming not. Have you thought of the potential CAN-SPAM and GDPR implications? Maybe just load the US data you know is U.S. as another filter if not?
Long advice short -- I would ensure your cleansing incorporated a few of the questions above, but when/if you load them into marketo make sure to flag them to both A) send them dedicated comms customized to people that don't know you, and B) leads you can easily remove from your database if there's no validation to keep them.
You might also look at Siftrock. they automate this and likely have some best practices you should go through when running this sort of campaign.
Definitely interested to see how it goes - please post your updates and successes here so we can follow along.
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