In the current portage tree, the mail-client/evolution and its components (gnome-extra/evolution-data-server, gnome-extra/evolution-ews) are very outdated. In portage is version 3.12.11 but upstream stable is at >=3.16.2.
Evolution's version numbers are confusing. evolution-3.12.x is the correct version for gnome-3.14 (in portage). evolution-3.16.x is the correct version for gnome-3.16 (in the overlay). And there was no evolution-3.14, because two months ago evolution's developers decided to go from version 3.13.90 directly to 3.15.91 and then 3.16.0. So if you want evolution-3.16, you need the rest of gnome-3.16 libraries, and you can get them from the gnome overlay. We are planning to move them to the main portage tree soon™. *** This bug has been marked as a duplicate of bug 545082 ***