I used to run my own XMPP server (OpenFire) and had my DNS SRV records correctly set up so people could chat to me from gmail. I found that one major source of pain was that Google Apps for Domains customers couldn't chat with me. It only worked with gmail addresses.

Why? Because Google Apps for Domains customers hadn't configured their SRV records for their domains. Why? Because for these Google customers, chat "just worked" with all other Google Apps for Domains and Gmail addresses with no configuration.

Google supported and used XMPP federation, but seemingly "embraced and extended" it to make it "just work" within their network.

I stopped running my system when I couldn't chat with my clients and didn't feel credible telling them they couldn't chat with me because their working but misconfigured chat system was in fact misconfigured.