Unsolicited untargeted bulk email.
Opt-In Email Marketing Vs. Bulk Email Spam
Spam is basically bad marketing ...via tricks, misdirection. Getting people to sign up for your travel newsletter then sending them a sales letter about your travel agency is spam. Coming up with a very bad newsletter to justify sending to an email address is spam.
Rationalizing sending badly written copy to people is basic to spammer mentality. Basically, if you couldn't sell your newsletter to subscribers, don't send it for free. It's not worth free. Basic spammer mentality is if people don't opt-out, then they find the newsletter valuable.
Spam is inept, badly written copy sent to the wrong people. To spammers, everything everyone sends them is spam, while everything they send is a finely crafted marketing piece.