1. An informative email message sent to a list of subscribers on a regular basis.
2. Spam messages sent on a regular basis to a purchased mailing list of non-consenting "subscribers".
3. An unsolicited spam message sent specifically to offend the recipients in order to provoke them into clicking an unsubscribe link that in reality enrolls them into mailing lists to be sold to spammers. See also spam riot.
Most newsletter recipients wonder how the heck they got subscribed to most of their newsletters.