How do mass mailers maximize their inboxing?

I am trying to understand how so much junk emails are getting inboxed in my emails. What type of setup are these marketers using to in order to deliver their emails. Is it a private server that spoofs IPs and headers? Or do they simply have an ridiculous amount of individual mail servers on rotation? Whenever we set up our own private VPS and domains, we always get IPs blacklisted and our domains get blocked. Our mailing lists are legit but we can't seem to inbox our emails. Now we have a freelancer willing to send 100k emails with guaranteed inboxing. How is this possible? What am I missing? Any advice is appreciated.

IT can be many factors by you are not inboxing..

You dont have proper rdns/dkim/spf etc setup,
You could have spammy words.
Your ips could of been blacklisted from the beginning.
You are not warming up ips.
You might need to modify your mail headers, the mail platform you are using could be flagged.
lots of factors contribute to inboxing.

If you mailing list is truly legit, go on a legit platform like Aweber

blackbeans said:

If you mailing list is truly legit, go on a legit platform like Aweber

Click to expand...

I have no problem utilizing platforms like AWeber but I still want to understand the process by which these other messages are getting through while ours are not.The issue with services like AWeber is that they may be cost prohibitive for a subscriber list over 50,000. With larger subscription lists, is the best option to run your own inhouse mail server and be your own ESP?