I want to be able to use 'Double Opt-in' instead of 'Single Opt-in' for subscribing to the newsletter. That is, when you subscribe to the newsletter, a message is first sent to your submitted email address requesting confirmation that this address should indeed be added to the newsletter database.
If the recipient of the confirmation request does nothing, then the submitted address is rejected (not added to the list). The address is only added to the distribution list if the recipient responds (click on a link) to the confirmation email.
Problems with Single Opt-in? Some people might submit their address inadvertently, and some may have their address submitted for them, against their wishes.
Does anybody else worry about email SPAM? This is basically what happens when your email gets added to a database without your confirmation.
Just about 100% of those who confirm stay with your service, product, newsletter... And they appreciate the fact that permission is asked up front. They are the ones that are willing to give a bit more in return for what they're getting.