So if they route everything through the same set of EC2 instances, that might actually take care of the SPAM issue. I run my own e-mail server and have run into that same issue of not being able to send to gmail/microsoft addresses (unless I contact the person via Facebook/Twitter/Reddit/etc. and tell them to check their spam folder):
https://penguindreams.org/blog/how-google-and-microsoft-made...
The way most of these providers deal with spam is they slowly white-list IP addresses. When a company like Mailchip or Mailgun spin up a new server, it's always in a large subnet range they've purchased and they slowly start sending low priority e-mail through it to existing/known receivers and throttle it up to full speed.
If you're running a personal server that sends like 5 or 6 e-mails a days, well that's an issue.
The big players make it difficult to run a small personal server, but running a dedicated business or corporate server that sends 100s of e-mails per day is typically fine once it's well established.
You know what would be a better product? A relay SMTP server that works with Google/Microsoft/Amazon/Fastmail et. al. to pump e-mail from personal servers and ensures it won't get caught in spam filters.