When the clients use the Register form, they are registered as Prospects and the Newsletter option is checked in their Client Information section. That only means they agree to be sent newsletters by you, not that they will automatically receive all of them. Then, only when you create a newsletter, you actually choose whom to send it to. The contacts in the groups you don't select (in COMMUNICATE TAB > NEWSLETTER > Create newsletter > Options tab) will not receive the newsletter.
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