> Marketing is only spam when it isn't previous customers, or people who have specifically opted in.

Yes, this excludes any people, customers or otherwise, who did not knowingly and willingly opt-in to specifically receive marketing emails / promotional emails / any other unnecessary emails.

A good heuristic is: if somebody receives an email from you that they do not want, there's a good chance you're spamming them: maybe by calling a marketing email, an "update" instead; maybe because you didn't make it abundantly clear to them when they opted-in that they would receive emails of that type.