You can use the unreal Engine for everything. I am sure there are some guys already developing an MMO. Buz unreals ServerModel is more for competetive games. Like FPS shooter or other simple structured servers.
The server system contains client and listen servers(the player itself hosts the server) or dedicated server(the server runs without a player and alle players are clients).
An MMO uses a completely different system. You would need a massiv server to handle the amount of data. You would also need an instance system, so that players can be splitted instead of overpopulating a zone.
Even if unreals server system could be changed to work similar to this, making an own server system for an mmo would be the best. You would also need a Database system to save everything.
BUT you can use unreal for everything else. You would only need to integrate your servers.
I don’t want to sound rude, but an MMO is one of the biggest projects you could start. It will take several months and maybe years. If you don’t know how to start and if you search for free system i would recommend you to develope something different and smaler. Also most MMOs by now arent fun at all. I played nearly every MMO and after a few weeks/months most players stop playing them. So even the AAA titles suffer from that. Please think about this and consider starting with a smaller project. Otherwise you may run into frustration (:
Thank you very much.I did not want to develop mmorpg by myself.In face I want to learn what need to learn while developing mmorpg beside ue4. then in the future I can MMORPG developing team in our company
There are some discussions about MMOs and server structures, as well as threads about databases and security. Not much, but i read some a few months ago. You would also want to check Google. Because you will need to use things outside of Unreal and integrate these into the engine later (like i said in my answer). So Google will give you more information on what things you would need to learn.