On of the many advantages of XMPP is, that everyone can setup his own server and everyone can communicate with everyone, like with Emails :)

Buddies → Add chat…:

https://en.wikipedia.org/wiki/XMPP

http://wiki.xmpp.org/web/Main_Page

http://xmpp.org/xmpp-software/server/

http://www.jabberes.org/servers/servers_by_times_online.html

• All systems: http://xmpp.org/xmpp-software/clients/
• Android: android:apps#a_gibberbot|Gibberbot