How to automatically accept the remote key when rsyncing?

If they genuinely are new hosts, and you can't add the keys to known_hosts beforehand (see York.Sar's answer), then you can use this option:

-e "ssh -o StrictHostKeyChecking=no"

You can add this host's key to known_hosts beforehand like this:

ssh-keyscan $someip >> ~/.ssh/known_hosts