interface Deno.MxRecord
If [`Deno.resolveDns`](../././~/Deno.resolveDns) is called with `"MX"` record type
specified, it will return an array of objects with this interface.
preference: number
A priority value, which is a relative value compared to the other
preferences of MX records for the domain.
exchange: string
The server that mail should be delivered to.