DeltaChat was much more useful when it worked with any email server and didn't need its own special kind of server. We have plenty of messengers that use their own servers, like matrix, XMPP, and simplex.
That was their one big plus and while I avoided it because of PGP, this means that it really has no upside over XMPP for me.
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."