In case not everyone has seen, you can go into Settings and switch on Enable Advanced Email Permissions. Whilst this doesn't tell you which email people have responded to, it does allow you to view and extract those with Express Permission. We found that if images are blocked on email delivery and the recipient chooses not to download them, they won't be included on the Open report, even if they've opted in. Which meant we emailed them a follow up - not great!
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