"Last read" / "new messages" line in conversation showing where you have read up to in a conversation
Similar to Slack / Microsoft Teams, it might be helpful to have an indication of where you last read up to in a conversation. So if new emails/messages come in, the last read line will appear below where you read up to, and above the new messages.
It might also be helpful (like in Teams) to be able to individually see read/unread status for individual messages, and to mark individual messages unread. This then moves the line, so you can keep track of where you're up to.
You could additionally have "Today" / "Yesterday" etc. as a line, to group messages in a conversation.