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isnt mass mailing just another name for spam
Not really. Mass mailing is sending a lot of mail. For example forum thread reply notifications, product updates, comment reply notifications, educational newsletters etc. Spam is sending unsolicited mail - usually commercial in nature. Spam is a type of mass mailing but not all mass mailing is spam. Sometimes people solicit mass mails.
A legitimate mass mailing list will double verify users.
Any user that wants to subscribe to the list or newsletter will first enter his email address on the subscription form. Before that user is added to the list a confirmation email is sent to his email address asking him to take some action to confirm his subscription. Only when he confirms his subscription is he added to the list. This way there is no doubt that the user solicited email.
Also in a legitimate mailing list the user can unsubscribe to the list at any time. Usually after he confirms his subscription he is sent a welcome message with instructions on how to unsubscribe from the list if wants to in the future. Every email sent from the list may also include unsubscription links in the footer.