1,970 votesjens commented
you develope outlook for android, ios and so on... why dont you stick to the standards, wich were developed to fulfill all the requirements. a good groupware solution with WebDAV, CalDAV, CardDAV is the way to go instead of developing new standards. And it would serve any device. Sabre brings most things microsoft exchange has. i have not tested it, but busycal and busycontact for mac base on DAV Services, which can be accessed by a lot of devices. http://www.busymac.com/index.html
or other groupware to base on. why is everybody talking about outlook? hardly anybody needs all functionsof exchange. most people might be helped with an internal owncloud solution, or if you want more sabre/dav, maybe with sogo or zarafa as an interface.