If an email has been replied to, would be great if there were some visual display indicating this to make it easy to keep track of, like a "Replied To" arrow or something in one of the columns.

In other words, if you get a lot of mail and reply to some but not all, then you have to check each one to see if you have replied to it.

Instead, if there were a Replied-To symbol there, then you can easily see that you've already taken care of that email and you can skip it and do the others.

Feature requested, thanks