You can use a 3rd party API or your own node to send (broadcast) your message (transaction) to a blockchain.
A JSON-RPC Endpoint is required to send transactions, a node is required for each network you want to send transactions on. There are different options for getting access to a JSON-RPC endpoint, we recommend using Infura, its a managed service and there is no cost.
Infura - Sign up for an account and create an endpoint. Each endpoint has a unique URL.
In each case the URL is supplied when initializing the Mailchain application.