I have not used Ubuntu Unity in quite a long time (despite me being actively involved in the community), so I don’t really know if this is applicable. However, I know that Evolution is (or has) been included on the default install for Ubuntu Unity. There is this new update for Evolution that includes CSD (Client-Side Decorations), and although I really like it, I don’t know if it will fit in Unity’s UI, especially with the gtk-noscd patch that it includes. Are there potential alternatives?
I know that Thunderbird is included on the default install as well, however, it looks a bit out of place with the redesign that it went through, though it still looks mostly consistent with Ubuntu Unity. Claws Mail, I think is a good alternative, as I think it looks more in place and it still also uses GTK3 and the old-fashioned appearance with global menus, so I don’t really think it will be moving to client-side decorations in the foreseeable future. Though, for now, Evolution is still perfectly fine with the gtk-nocsd patch, it still looks in place, though I don’t really know about its future, especially with many developers heading to Geary and the adoption of libadwaita.