About what are you speculating? It's not at all clear.
is the "edit pop account messages" window an option in a hidden Debug menu similar to the one in Disk Utility?
It's not an option I remember having enabled on my own, and if Mail has got a debug menu, I'm unaware of it.
Sorry, I've got absolutely no idea when or where I learned the trick, but I can say that I've been using it for years. (I mentioned POP, because that's what I use; I think the window also works for IMAP.)
Edit: I just looked in my plist to see if there was a relevant entry, and he only thing I found was
<key>AccountInfoLastSelectedAccountId</key>
<string>1170d37e-0799-4ef4-bea3-d59569952851</string>
("Account Info" is the heading in command-i windows.)