Opt-in
Also known as: single opt-in(soi)
With opt-in, the owner of an e-mail address has given explicit and provable permission to receive e-mail from a particular mailing list. The e-mail messages are wanted, as opposed to spam.
Of course, this form of permission is not foolproof. It is easy to enter a nonexistent e-mail address or provide someone else's e-mail address. This can clutter the recipient list with incorrect or uninterested recipients. To prevent this, many organizations today use double opt-in.
When a recipient is automatically placed on a mailing list (without prior consent), it is an opt-out.
