Here you can find some recommended Jabber/XMPP service providers that offer to host an XMPP server for collective use by a group or organization. They also allow the collective to use its own domain name for Jabber IDs like this:
Conversations.im is the commercial offer by the developer of the Conversations App. Learn more about how to register and link your own domain name on their website. Their services are hosted in Germany.
Hot-Chilli is run by an IT company from Germany and has been running a free XMPP server since 2005. You can learn more about their Jabber service here. It is possible to connect your own domain name to their XMPP server.
Snikket is a project started by one of the Prosody developers to provide a unified app experience and user friendly server hosting. Learn more about their goals here.
The developers of the Tigase XMPP server also offer professional server hosting for large organizations.
This is obviously not an exhaustive list and if you have a really good recommendation please contact us here.
If you are the server operator of one of these services and disagree with being listed here, please contact us as well.