I have Apple Mail version 9.2, but I would like to have the version 10.3. However, I don't want to download the 10.12 mac OS (Sierra) if I don't have too. I currently have mac OS 10.11.3 (El Capitan).
There is a discussion here involving a much older OS (tiger) in 2010: https://discussions.apple.com/thread/2502383?tstart=0
I can't find many other resources discussing upgrading mail.