I am worried that the raspberry pi can't handle running owncloud, a mail server, ftp server And a dlna server at the same time.
Why are you thinking this way?
That's not correct.
I have running owncloud, (s)ftp Server and dlna Server at an 256MB RAM RasPi. And it works fine. I'm sure, an additional mailserver will also work fine.
It depends on the number of clients using those services. Running as an home server is absolutely no problem.
Do you think i can have the apache and ftp server running one one RasPi and the mail server on another (the mail server also has to run an apache instance for a webmail interface)?
Sure. You can setup every service on an single RasPi.
I don't know, how good is your knowledge in server cluster is. The point is, that some peoples try (would like) to "patch" some machines together in order to get one big machine which has much more power.
Building a cluster depends (in most cases) on software which handles the services, sometimes hardware does it.
Easy construct: a 2 node webserver cluster and one NAS as central storage. For this you need "something" which redirects the users webserver request to node 1 or node 2 (which can be done by DNS Servers, Loadbalancers, switching IP, ...)
But the point is, on an 2 node cluster you will not have the performance of an Dual-CPU RasPi, because your request doesn't scale over nodes.
Or an other example as @Wendo wrote: video encoding in an cluster. Take one server as masters to start encoding and (mostly) this servers splits the video into small parts in order to transfer those parts to an other machine to start encoding of this part there. In this case: again an software cluster.
Btw: all of the real big clusters i know are working with software (...and maybe loadbalancers). So once again the question for you is: which services you like to provide and which cluster methods are the best one.