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I was testing out the list handling in Sendy and setup another list where I tried to subscribe an address. It refused to subscribe it but without any error, initially I thought there was a limit where you could only have a certain e-mailaddress in one list, which seemed odd. But when I tried it again through the API it worked.
How this is handled through the admin interface probably should be fixed, either it's a bug that the address is not getting subscribed. Or there's a message missing to tell the user why the address is not being added.
This is a deliberate feature a lot of users have been asking for. It's not a bug. You shouldn't be able to import the same email address again in any list of the same brand if they previously unsubscribed from your newsletter.
If you want to change this behaviour, change the 'Unsubscribe settings' of the list to 'Only this list' (see screenshot below):
Ah, thanks for the quick reply and thanks for clearing it up. I was looking for a setting but completely missed it. The confusion came from the inconsistency between the admin interface and the API.