Sort Apple Mail by "replied to" I'd like to sort my email on whether I've replied to it or not. Apple Mail puts a little replied to icon to the left of whatever emails I've replied to so I know this filter exists. 

Thoughts on whether this is possible?
Although Sorting by reply status is not possible, you can certainly Filter by reply status.

Using Smart Folders you can collect all replied to messages in one 'folder' and unreplied to in another.  These folders just gather the messages, they do not move them from their original folders unless you delete them.  But this can help you keep mail sorted.

Then you can sort by date or other desired criteria.