Freebsd, postfix and push email

[Ron]

Jeffrey Goldberg wrote:
> On Mar 28, 2010, at 1:36 AM, perryh at pluto.rain.com wrote:
> 
>> Er, no.  POP3 and IMAP are "pull" services, wherein the client 
>> polls the server periodically for any newly-arrived messages.
> 
> IMAP, but not POP3, can be used to push, but the iPhone mail client
> doesn't support that as far as I know.  It does support being pushed
> to over Mobile Me, but not on regular IMAP.
> 

So how is Mobil Me and Exchange Servers (MS, Zimbra, etc) doing it?  That's what I'd like to replicate on my FreeBsd server.  This seems to be a well guarded secret I'd like to crack.  

On the iPhone, at least, the phone is not polling the servers, some kind of message (SMS?) is being sent to the phone that makes it put a little red badge on the corner of the mail app icon telling it how many messages are waiting.  The messages don't seem to be download during the push, it's just a count (I could be wrong about this).

-- R