The nature of an email newsletter which aggregates third party content is that the only target goal is something opening your email and then enjoying the content enough to click to read more about it.

I don't necessarily like the model, but if this is your business model, then there's nothing else that can be measured to monitor ongoing success apart from:

- how many new users signed up

- how many existing users unsubscribed (or marked as spam)

It's a tough ask getting sponsors based on those two metrics alone.