Most of us are inured to email spam, and it's hardly surprising that there is so much of it, since email is an open ecosystem.
Loosely speaking, anyone can run an email server of their own, or an email client, and connect to any other email server to inject good, bad or indifferent messages into the email universe.
If you don't want to run your own server, there are thousands of organisations that will offer you space on their servers, from webmail giants like outlook.com and Gmail, to conventional email providers such as your ISP.