While there are other metrics, to begin with the two most important metrics one should track everyday is the Open Rate and Click Rate in order to measure the efficacy of newsletters.
a) Open Rate is one of the best ways to gauge whether your email strategy is working. This number shows what percentage of your audience opens the emails you send them. A great open rate, it usually means your subject lines resonate with your audience. The average open rate is 21%.
b) The Click Rate is a percentage that tells how many successfully delivered emails got at least 1 click. This number shows whether or not audience finds emails relevant enough to click through for more. The average click-rate is 2.43%.
Further to it are, Bounce Rates and Unsubscribe Rate.