Continuing the discussion from phpList: import emails tutorial:
I think that the Invite plugin is now meant to handle this situation. You can send a campaign to the imported subscribers that asks them to confirm their subscription. The subscribers are marked as blacklisted so that if they do not confirm then phplist won’t send to them again.
There is an outstanding issue with the plugin though, which means for phplist 3.0.11 and 3.0.12 when a subscriber confirms they are not sent the “welcome” email. I’m not sure whether there is a pending solution for that.