For years, I have helped companies increase their newsletter subscribers through marketing tactics as well as ensuring strong, relevant content.
Currently, I have my own newsletter - Elevator Sports Talk - that gives a very simple download of the sports headlines for people who are too busy to read the sports pages or keep up with the on-going drama in sports.
So, I pay a lot of attention to how to increase subscriber counts and visibility. In particular, I watch strong newsletters that take off, who's behind them and what can I learn.
Here's a great interview with Dan Primack, the business editor at Axios, and his thoughts on "authoring a popular email newsletter."
Mainly, he cares about putting out info that others haven't.
"I want to tell a reader something they don’t know yet ... I want to make them feel smart when they get to work and feel like they’re ready to do their jobs.”
Good interview listen: https://www.recode.net/2019/1/17/18185863/axios-dan-primack-business-economy-ipo-pro-rata-newsletter-media-podcast-interview-peter-kafka