The way I do this:
1. Enthusiastic opt-in... it's not even advertised (only 65K members), and you get it on via receiving more transactional updates from a few forum and a footer link gives you the option to receive a broader summary too. It's very strongly interest aligned, and by the time someone discovers this it's niche and highly relevant.
2. Least tracking possible + simplest HTML possible (easy to read, accessible, easy to open links even if you aggressively adblock / trackblock).
3. Trivial opt-out as I don't want to send to anyone not interested in it (it costs me money to send).
4. Time. Building an audience like this just takes a long long time (12 years now).