Artie, this may be a red herring as I am using MacOS as you are, but I did a search for com.apple.mail.plist using FindAnyFile and to my surprise found several on my boot drive. Then I started opening them in Xcode and found the contents were very different and the one in my user Library was almost certainly NOT the property list plantsower is looking for. I found the one I believe you all are looking for is:
~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist
When and why Apple has moved it and why they have chosen to use such a baroque file path is well beyond my ken, but there it is. You have done a great job guiding plantsower so far and I am backing out and leaving it up to you to determine if that was the case in Sierra or a more recent change.
PS: That isn't the only thing that has migrated to a new home in the ~/Library/Containers folder.
