If you bought a domain and you do not want to buy a web host for doing your email service or you are doing a service which is only providing website hosting wihout email service such as shopify, then the simplest solution is email forwarding service. Recently, I found a free one ImprovMX. They are free to up 5 domain and 10 alias. You only need to point MX record to their server and put TXT record with a random string to verify the ownership of the domain name. That is very easy to use!