Do you mean importing a purchased list for emailing? Not Data.com or zoom?
There are some threads on this someplace.
I would say that, if this is legal in your jurisdiction, and the vendor is not emailing on your behalf that you
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dedupe against your list and suppression lists, which Marketo will do automatically.
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That you tag imported emails somehow - Static List is default, but maybe Lead Source, etc.
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that you separate US emails from foreign. I believe only US is opt out and everyone else is opt in.
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if they don't respond, I'd be inclined to delete them from the system. definitely check the bounces on this one
Usually these types of lists lead to low deliverability, so be careful not to hurt your rep!