codexon did a pretty good job but it's little more: the three main consumer email providers/hosters (google, microsoft, and yahoo/aol) don't have a great way to fight spam so use IP reputation, what this means is that if the IP address that you use to host your email server doesn't have a good reputation they will silently drop emails you send to their users and neither the recipients nor the senders (you) will ever know.

Only real way around this is to instead host your email server with them, aka Office365 or gWorkspace. This basically breaks the federated aspect of email and turns it into a siloed SaaS offering from two of the big cloud providers. AWS can also be used to send newsletters but doesn't really do consumer email hosting (they don't have a good client), and you're still tied to a SaaS cloud provider.

The view is that most emails are becoming centralized to come from a few places. So email hosts like microsoft and google take shortcuts to stop spam by untrusting any IP they haven't seen sending emails for 5 years (this is just a random number, no one knows what the real filters are).

I work in a company that sends millions of emails every month through our own server. We moved to new IPs last year and didn't see a big disruption. The key is having a high quality list and only ever sending things that people actually asked you to send.