XMPP
XMPP, originally Jabber, is a decentralised messaging service which originally came to light in 2004 and was one of those messaging systems I recall using back in my teenage years, when the messaging landscape was fractured and multi-protocol clients like Adium and Trillian ruled.
When Facebook messenger decimated the chat landscape, XMPP didn't go away like its contemporaries did. Instead, it lay dormant, and when the current wave of interest in decentralised protocols surged again, it sprung up like a particularly effective weed.
As with Matrix, XMPP is a protocol, not a platform. Unlike Matrix, the body who maintains XMPP (the Internet Engineering Task Force [IETF] and XMPP Standards Foundation [XSF]) doesn't build a flagship client. This, along with XMPP's age, means there's plenty more clients out there for XMPP than for Matrix, as there's no obvious "default" app. Similarly, users are more dispersed among servers, as there's no default central server.
The XMPP protocol comprises:
- A core specification, maintained by the IETF.
- A series of extentions known as XMPP Extention Protocols (XEPs), developed through the XSF.
As a basic chat replacement
Using XMPP is easy:
- Get a client for your operating system.
- Pick a provider and register your address.
Your XMPP username takes the form user@domain, a la email.
As a discord replacement
XEP-0503 defines XMPP server-side spaces, which allow users to group together multiple chatrooms. This allows you to build an equivalent to Discord's servers or Matrix's spaces.
This protocol is currently (May 2026) marked experimental, which means it's not widely supported (on clients at least, I believe it's supported on ejabberd, a major server implementation). Movim has experimental support for the protocol, but while it's experimental it's likely most clients won't support it. There's hope that servers and cliets will begin to support it as its status solidified.
There's a bit more information about the impetus to create spaces, the process around getting them to the current state, here.