I tend to measure success of an email campaign based on email engagement via clickthrough rate.
There are 3 metrics that I look at for each send:
1. Clickthrough rate: shows how much the content we are sending is resonating with our audience
2. Unsubscribe rates: how much the content is not resonating with our audience
3. Influenced monthly recurring revenue 90 days after send. We measure this by looking at clickthroughs and then deals closed within that 90 days after recipients open and click the email.