What is the difference between cold emails and spam?

I'd consider a cold email an email sent to me because in some way I could be interested. Also, I'd not be bombarded by many other following email unless I'm not showing any interest (like click on some links or confirming I'd like to have more emails like them).

Then, I'd consider spam all the emails coming constantly, like after the 3rd one, for a service that I'm not even interested; but more in general, constant emails that I didn't ask for.

I would say scale

Personalisation. This and this are good examples that should make it clear for you.