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.