There are many conflicting definitions for spam circulating in discussions today, with almost no consensus. Forrester defines spam as any unsolicited bulk e-mail in which the recipient either is unable to establish that a relationship exists between he/she and the sender. The ability of the recipient to avoid receiving future messages from the sender is often referred to as an “opt-out” feature. Spam is not defined by relevance to, or interest by, the recipient by the recipient, but by offensiveness to the recipient.”]
Source: https://www.csoonline.com/article/2116806/defining-spam-is-the-first-step-in-stopping-it.html