The thundermail domains are thundermail.com and tb.pro. I'm curious to see how they will compare to the top privacy-respecting email providers today, and how they think they will "provid[e] a better service than the other providers out there," which would include Protonmail, Tutanota, etc.
The issue of centralization can be a problem, but in regards to metadata, sealed sender does a lot to prevent Signal's servers from knowing who messages who, which makes Signal a lot more private than described here.
No, seriously, what's up with those messages 😭