Thanks for the suggestion, which sounds good to me. When I import the static list of email addresses, would this create a new person entry for each listed email address in the main database? Because that's something I would like to prevent if possible.
To be clear, it will create a person in your database.
Now, if you want people to be Unsubscribed (not the same as Blacklisted) you can import people, set them to Unsubscribed=true, and then delete them. Marketo will remember that they were Unsubscribed when last seen, so if they come back into the system they will start off Unsubscribed. This is known as the "Durable Unsubscribe" feature.
However, DU may not be completely appropriate for your use case — it's a not a permanent setting, as Unsubscribe can easily be un-set for people on the DU list afterward.
You might also think about a webhook-based solution. That's how we manage massive suppression lists (like 100s of thousands): they're stored in multiple Google Sheets (one for each do-not-send source) and then when someone enters Marketo we check to see if they're in one of the lists. This avoids the database churn of adding, then deleting people.
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