The default mode of Delta Chats "email interaction" setting is "show only chat messages". This includes replies to messages you sent out from Delta Chat, even if the reply came from another email app or a webmail interface. So you can chat with anyone who you know the email address from.

If you set email interaction to "all" then you see regular email appearing as a "contact request" if you haven't already accepted the sender. In current development repos there is already support for mailing lists, and for html-mail. With this and a few other improvements, delta chat can be used as an e-mail client in more situations.