After a user has subscribed to a newsletter, they will receive an email that will contain a link to confirm their subscription.
If the user has not yet confirmed their subscription, then the contact information will be displayed in grey, with a notification stating that the contact has subscribed but not yet confirmed:
As seen in this screenshot, the second contact has been verified, but the first one contact has not yet done so.
In case a user still needs to confirm their subscription, but is unable to find the original email with the confirmation button, then they can simply subscribe again through the newsroom and they will receive a new confirmation email.
It's also possible to filter your contacts to show everyone who has this 'pending verification' status:
When trying to add a contact yourself that was already added but hasn't confirmed their subscription yet (pending verification), you will see an error message explaining why this is not possible: