For example we have many clients with domains where they have full mail services under their domain but it would be highly convenient to add (or even in a rare case override e-mail addresses) without the need to add them to their full e-mail service. But it would not work to have Cloudflare be the only MX. Cloudflare would have to be smart enough to be the proxy for all MX traffic and then route on those messages that are not routable directly by it. If it is this smart it would be very cool.
When Email Routing is configured and running, no other email services can be active in the domain you are configuring. If there are other MX records already configured in DNS, Cloudflare will ask you if you wish to delete them. If you do not delete existing MX records, Email Routing will not be enabled.