Email and DM is different though; email is message centric, DM is relation centric.

The base view of email is every incomming message listed in chronological order; every message is its own thing (aside from that it can have a subject); the people involed (recievers, cc, bcc) are all liked to the message. In DM its the other way around, there messages are linked to the leople involved.

So if you are making a new thing/kind anyway, and your idea behind it is 'email', then please just copy the most basic bitch email UI pls. It also sends a signal to the user in terms of expectations; in what timeframe people might respond, formality, formatting, attachments etc. There is all this implied etiquette differences surrounding email vs DM