We use a self-hosted roundcube mail server, which is not necessary, and we did initially assign each person an additional account dedicated to delta.chat to prevent inbox clutter, but that didn’t happen.
Nowadays, we prefer to send and receive emails in delta.chat. One of my groups already has 70 members, contains a lot of images and PDFs, and it’s still very fluid to use.
We also run bots for integration with other systems, such as task management, meeting notifications, etc.