Inbox Rate is directly proportional to an internal algorithm that (gmail, yahoo, aol, hotmail, ...) use. It works something like this.
1) Have we ever seen this IP before (If yes, are people opening these emails, deleting them, clicking spam...checking engagement)
2) Have we ever seen this FROM address (If yes, it's similar to #1)
3) Does the content of the email have spammy keywords
4) Is the sending IP blacklisted
5) Is the sending domain blacklisted
Think about it. Do you think gmail.com says "I'm going to inbox this XXX offer from a BRAND new IP address and unknown sender to thousands upon thousands to gmail.com users"? They are smarter than this. Once you start thinking like gmail/aol/hotmail/yahoo, you'll start winning the game.
Everybody wants to send millions of emails. I don't understand why. I'd rather send 5,000 emails if i knew the open rate was >70% than sending to a million with a .01% open rate. You know why? Because in the long run, I want to consistently make money and I want to inbox. You'll also learn that there are HARD limits to the number of emails that you can send to each provider an hour per IP. So, you want to send millions of emails out in a day?
1) Run them all through Brite Verify (that'll cost you $0.01 per record)
2) Buy a /24 (256 ip addresses)
3) Configure PMTA to properly backoff based on the sending algorithm
4) Find a platform that will handle this amount of sending (and one that plays nicely with PMTA)
5) SETUP YOUR FEEDBACK LOOPS (remember once you hit that .1% complaint ratio, you'll start hitting the SPAM folder, then you'll be 4XX blocked, then 5xx and GAME OVER)
6) Send slowly
Plan on spending at the very least $500 on getting this up and running. Then ask yourself something. Did I just spend $500 to spam some shitty list that I bought/found and it's full of spam-traps?
If somebody is trying to sell you a solution for bulk mailing, or claims that they can inbox your scraped emails...RUN FROM THEM! They are just out to take your money, not to help you grow your business.