It defeats the selling point of DeltaChat. You may as well use XMPP or Matrix.

It still works with regular mail servers:

Quote from https://providers.delta.chat:

"Usually, it just works, but for some providers, you need to adjust the settings. In this case, click on “prepare” for details.

Please note that push notifications do not work with classic providers and esp. large classic providers often have sending limits and overzealous spam rejections. This can worsen the chat experience, as messages are delayed or missing messages may even disrupt groups."

I've used both. Neither are as good. Your reply also completely ignores my point about the ease of hosting the infrastructure yourself. Prosody isn't too bad, but hosting Matrix is a nightmare -- as much as I admire what the Matrix crew is trying to do, the architecture is unwieldy and always has been.