To send testnet messages using Mailchain, you will need an Ethereum address with a small balance (around 0.0007 Ether).
Ethereum addresses are valid across the Ethereum mainnet and testnets, so it's best to create a new address for development purposes only. This way, if your private key gets exposed, only test ETH can get stolen.
If you are using Metamask, you can switch networks in the Metamsk interface and go to https://faucet.metamask.io/
MyCrypto or Other Wallet Software
You can head to one of the following faucets and follow the instructions to be sent some test ether.
In the Mailchain Inbox, switch to the testnet that you want to send a message on (and have a balance on).
If this is the first message or transaction you are sending on this testnet:
Click "COMPOSE" in the Inbox interface and send a message to an ethereum address which has already sent a transaction, e.g. the Mailchain Developer address: 0x4ad2b251246aafc2f3bdf3b690de3bf906622c51.
If you have sent a message or transaction on this testnet, you can receive messages from other developers, send messages to yourself, either from the same address or another development address.