Unfortunately, currently, there is still no way to start a chat if using a classic email client, yet.
So deltachat users can not be reached by known contacts (those, one has exchanged emails with before).
The only workaround currently, is for the deltachat user to manually check for your message in the email inbox (not nemed well as “Contact Requests” in the menu).
The following is a suggestion that amoung other nice things, should also allow receiving certain email messages in a chat: