I’m not someone who’s in charge of the Delta Chat’s philosophy on this front, but:
Not sure if this is much about the Delta Chat clients rather than the email providers, namely the now pushed for “Chatmail”. If you use Delta Chat with a regular email provider (such as Gmail, Outlook) and not Chatmail (e.g. the default Delta Chat provider), then you do get messages from spammers, as “Contact Requests”.
There are some ideas about having contact requests in a separate section, or showing just one notification when you get a contact request, instead of doing that on each message.
See more about the common email spam and Chatmail in the blog post below. In short: if they don’t have your invite link / QR, they can’t message you.
With Chatmail servers, there is a difference between having someone’s email address and having someone’s QR. If they only have an email address, they can’t message you: the server will reject unencrypted email.
The idea is to just not post your QR / invite code publicly, evident by the fact that there is an “Only share it with the people you trust” warning when copying your invite link. See a bit more about this in FAQ - Delta Chat and Why can I only add friends using QR codes and links?.
If you have posted your invite link publicly already, there is a button to “reset QR”, which will make it no longer work, i.e. people cannot get in contact with you using that QR / invite link. A new QR will then be generated which you can then share with “the people you trust”.
Another “tip”: some people utilize multiple Delta Chat profiles / accounts for various “trust circles”. Some more public, some less.
Perhaps somewhat related: