From DHH's long-winded keynote at Rails World 2026. He was bragging that Hey's backend is so efficient because it's written in Rust (which he hates and refuses to read).
I do appreciate some of the philosophy of Hey, like cherishing the human-to-human section of your inbox. But it's simply not worth the price tag. Also now that DHH has proudly proclaimed he does not read any of the code, I don't think it's even worth it for free, if I'm gambling with my data. I just self-host Stalwart and Bulwark.