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For some reason the unopened list can only be exported for the most recent campaign. This makes no sense. We have campaigns running to multiple different lists at once, and it is useful to be able to review the list of unopened emails even in the future. I realize I can export the opens and then manually cross reference, but this seems like a needless step.
If you want to send another campaign to ‘unopens’ only for any campaign, alternatively, import subscribers who 'opened' your campaign into a new list from the campaign report:
Then exclude this list when you send your campaign using the ‘Exclude list’ feature. Emails will be sent only to subscribers who didn't open.