When I asked my cousine, working for a large and well-known company in New York, that why don't she uses their own IM system, just gmail. She told me that they don't use an own IM system.
- Do you use blackberry?
- For Push-email?
- That's IM.
Come' on: it's Instant and Messaging. It does not have presence subscription, but it does have a roster (it's only called Address Book). Surely it has advantages (people who "has an older version of internet installed on their computer" can communicate with you), and there are disadvantages as well (they don't see how busy you are, therefore they can't decide if it's important enough, something which could be solved with presence).
But before trying to hurry your marketing departments to advertise XMPP as a free alternative of Blackberry, you should first migrate from the old system. What am I thinking about?
- Write a component which is compatible with push-email, so they don't loose functionality
- Write a component which is compatible with postfix, qmail, whatever smtp servers are popular nowadays, to work together with XMPP servers - this way creating an XMPP-based free push e-mail alternative
- Write a client, preferably for mobile phones where this transport could be handled conveiently.