How can I specify where Apple Mail windows open on the desktop?

Hello - I have a Mac Air with an auxiliary Thunderbolt display. I usually keep Apple Mail running on the Mac Air display off to the side. When I want to read an emailor reply to one, I double-click on the message to open it in a separate window and then, often, enlarge the typeface. I would like to have these individual message windows open in the middle of the big display right in front of me and this used to happen seemingly by default when I was using an earlier version of OSX (10.7, I think). Then I upgraded to Mavericks and this doesn't happen any more. How can I make it do that? Thanks.