We occassionally use Emma to send to a list of contacts interested in participating in surveys, but otherwise they are not our target audience or prospects.
It's important, however, that after any external send I export the list of unsubscribed leads from Emma, and import them as opted-out in Marketo just to cover my butt and make sure someone who's opted out remains opted out.
Why are you considering this for your opt-in mail, what are you thinking the benefits would be? Never importing someone who isn't interested?
I really like the ability to run subsequent campaigns on leads who made no response to an opt-in mail - keeping dripping content and poking for that confirmation. Similarly, even if someone does opt-out, their data is still in Marketo (and Salesforce) so our sales team can attempt to follow-out via phone, or at least has that contact information when trying to drill into a company.
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