It sounds like XMPP (also called jabber) is what you need. XMPP is an open standard for instant messaging, and there are free/open source implementations for both clients and servers. One option for servers is jabberd. One option for a client is Pidgin (which runs in Windows and Linux).
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Bah. Sorry for the google search result link. Here is a direct link to microsoft.
All the "Activesync Protocol" is, is good old PPP.
Not even close to the same universe as correct.
ActiveSync is an email synchronization protocol (among other things) built on top of WBXML and HTTP(S). See Microsoft's documentation for it here.
Can you give examples of good Exchange replacements?
Have a look at the PostPath Email and Collaboration Server. It is a mail server that runs on Linux and acts as a drop in replacement for Exchange (i.e. it implements the MAPI protocol so no special plugins are needed). They were recently acquired by Cisco, so I'm not sure what that has done to the availability of their server.
Admitting my biases, I am former employ of PostPath.
For some time now I've been using >=3 as an emoticon for "hate". Rarely do people seem to get it, though.
What barriers do you propose might exist that prevent one ancestral population from diverging into two arbitrarily-different ones?
If an individual strays too far genetically, God drops a rock on it.
Is that rock so heavy even God can't lift it?