Mailbox providers consider many things when deciding whether or not to deliver your email to the recipient's Inbox. One of those things is recipient engagement.
There are two main components that determine engagement:
How your subscribers like your content and mailing frequency.
How clean and valid your subscriber list is.
Obviously, if you aren't sending to real people, then there is nobody to engage or act on your emails. And, more importantly, it's a signal to ISPs that you're not a conscientious sender.
So, how do you build a valid email list of engaged users?