SMTP Relays are third-party services that can deliver email on your behalf. They can be useful when, for example, port 25 is blocked, the cloud provider/ISP doesn't provide Reverse DNS, or the IP address has a low reputation, among other situations where deliverablity isn't great.
These services are governed by their own terms and as such limits can be imposed in the usage of those services.
Here, you can configure an authenticated SMTP relay and authorize it's associated servers to send mail for you.