Perhaps I should also add that DeltaChat is really just an email client with a user interface that is optimized for chat, and the chatmail servers are just IMAP/SMTP mail stacks optimized for DeltaChat.
DeltaChat uses AutoCrypt to provide gpg/pgp encrypted “email” + metadata, and the chatmail servers are set up to refuse unencrypted messages (thus avoiding the spam problem).
The optimal experience is acheived by using DeltaChat the way it was intended: DeltaChat to DeltaChat communication, and I'd say that using the chatmail servers is also preferable to using custom IMAP/SMTP servers (whether they are hosted by the big providers, smaller providers, or self-hosted). You _can_ use DeltaChat to communicate to email-only recipients, but it provides for a sub-optimal experience.
People tend to have all sorts of reservations against DeltaChat, even if they haven't tried it, and I just haven't experienced those problems.
I have tried DeltaChat with my own mailserver, and with Gmail, FastMail and DeltaChat's chatmail, and it has worked flawlessly. If it hadn't, I wouldn't be a fan.
This is obviously only anecdotal, so other people may report other experiences.
If used as chat only, and with dedicated chatmail servers, the number of theoretical problems diminishes significantly. Using it as an email client and/or with standard email providers introduces potential points of failure, but not more than email to email.