Email has threading support. I can't find such feature in other communication tools
Note that threading support is not a particularity of email itself, it's "just" a set of headers that have a well-known meaning. There is no functional meaning from a MUA or MTA point of view. Any communication tool can implement the same, it's all in the client after all; and sure enough, XMPP has a XEP for this (http://xmpp.org/extensions/xep-0201.html).
Thanks for that, rakoo.
Do you know any XMPP client that supports threading? I am using Pidgin (for irc, facebook, yahoo), skype and viber. None of them has the feature.
I don't know any XMPP client that uses it, probably because threading is really useful with asynchronous communication, which XMPP hasn't really explored.