Anyone can send 5mm emails, it's another thing to inbox.
I've been mailing for over 17 years and most people have no clue how to do it.
For the most part, it all about data quality. The main problems you will have are complaints and lack of engagement.
The key to getting around this is to initially work with the best data you can get. You want to clean the data using a respected data hygiene service.
I use fresh / targeted 3rd party data, which is email data that users have shown an interest in a specific niche or offer and provided permission to receive messages from third parties. You get the opt-in record of all user and it's 100% can-spam compliant. I also generate real-time targeted co-reg (opt-ins). This is more expensive than 3rd party data, but you don't have to clean it.
Using the above data you want to send it using ESPs / SMTPs. This is because you can piggyback off the good IP reputation and get decent inboxing.
At this point, it's all about building good data (opens / clickers). Using 3rd party data you will get some accounts terminated. With co-reg you shouldn't have this problem as it is basically opt-in quality.
Eventually, you will be able to take your open / clicker data and use it as a buffer to offset complaints when testing new data. You can use it to warm up IPs if you want to run your own server / IPs.
Once you can offset complaints you can ramp up the volume testing new data / building more opens / clickers...SCALE
You can also buy open / clicker data, but it's not the same as opens / clickers you build yourself. But the users have shown that they will open / click on commercial emails. So it can be beneficial to use.
Most of the services that you see that offer servers / IPs and say you can mail all you want with no problems are just taking advantage of new mailers because they don't know any better and totally jack them on the price. The real companies that provide IPs and are mailer friendly don't make it openly known.
Anyway, there is obviously more to it, but that you give you some ideas.