When replying to emails, I've often needed a list of all emails from the sender to keep track of the history of our conversation.
The most convenient way, I think, would be an additional list with the history of emails in a frame in a side pane.
If you click on an email in the inbox, the history should show all emails from the sender, and all emails I sent to the sender.
If you click on an email in the Sent and Drafts folders, the history should show all emails from the recipients, and all emails I got from the recipients.
There could be filter buttons like "CC" or "BCC" in the history list and at least a date column.
When you click on an email in the history list, it should become the currently displayed email in the default pane.
I think this would be a very nice feature that could increase the productivity of heavy email users and provide everyone with an easy overview of their conversations.