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Double opt-in aka confirmed opt-in: In this method of email list management a confirmation message is sent to the subscriber who signs up for a particular email list. The subscriber has to click on a link in the conformation message to activate his address to that list.  In this way the email address of the person is verified and hence the name confirmed opt-in. Many companies use this method and nowadays it is considered as the most effective way to manage your email list. It avoids the possibility of any sort of spamming.

Single opt-in aka unconfirmed opt-in: In this method there is no confirmation message sent to the subscriber. When the subscriber adds his email address, the email address is added instantly. This may cause serious issues as there can be typo in the email address and in that case the email will be delivered to the unintended recipient resulting in spamming. So, unintentionally the spamming will happen and the list owner won’t even come to know as to why the personn has filed a spam complaint.