- No deceptive information
- Valid physical address
- Must have an easy way to unsubscribe, most often an unsubscribe link but asking the recipient to reply to stop being sent emails is also sufficient
- Promptly honor unsubscribe requests within ten days
Other countries have different laws though.
https://www.gmass.co/blog/is-cold-emailing-illegal/
> As long as I cost them enough to make sure they'll never even think of contacting me again, I'm happy.
Honestly if they're following the above rules, you won't cost them anything. And if they're not, and therefore doing it illegally, well, looks like your lawyers will have their work cut out for them.