Second issue of Basic Problem just went out to 62 subscribers. Last week's edition had not a single bounce, no churn yet, and an open rate of 30%.
Amazing to see that the subscription count almost doubled from last week, taking into account that my main referral source is still just a low-cost Google Ads campaign. Frustrating thing is that I could have hundreds of contacts in the Mailchimp Audience. But only a fraction of sign-ups finish the GDPR-imposed double-opt-in step. Some sign-ups might be bots, some misspelled email addresses, rest probably got eaten by a spam filter.
I also added my newsletter to most directories from https://www.indiehackers.com/post/list-of-newsletter-directories-dfb6499dfb, but visitors from those sites are rare yet.