I "built" a newsletter app
You can use it to read, archive and organise newsletters. It's free, synchronised between your devices and open-source.
OK, built might not exactly be the right word. I wanted to build one, as I've become quite a fan of newsletters and have quite a few subscriptions. I'm not so much a fan of them clogging up my inbox every morning, which I regard much more as a to-do list and not an inbox for newsletters to be read when I feel like it.
Thus, I was already working out how I would do it, maybe an email that you can send them to which would present them to you with good styling, and allow you to save some as your favourites for later. I promptly noticed, that email had all those features and more. I could achieve all this much more simply by just creating a new inbox, and use my time for the projects I wanted to focus on.
So here is my amazing newsletter app. It consists of logging into my email host (which luckily allows me unlimited mailboxes) and creating a new mailbox. I changed my email for all newsletters and installed Thunderbird on my phone and allowing it to display external images, so the newsletters would look nice.
Done. I now have the newsletter app I've wanted the last few days - I can archive newsletters I may want to reference later, pin them or even make a folder/"tag" structure.
Going Further
It might still make sense to build a purpose-made experience/app, maybe to apply a custom reading theme, strip out trackers, etc., and maybe I will one day.
I also very quickly thought of an email-to-pdf-to-email service, i.e. your newsletters get sent to your kindle as a PDF. I'm still a very big fan of this idea, especially as it probably has some applications beyond newsletters. Maybe I will build that in a not so distant time.