This has long been one of my biggest complaints and wants from CC. It is amazing to me that they still do not have this feature! I send out emails for an upcoming event with a registration link. I want to exclude anyone who has already registered, but removing that email manually from all my lists is unreasonable. So people who have already registered end up getting multiple emails and some of them register more than once. Simple suppression functionality, where I can upload a CSV of emails NOT to send to, would solve this!
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