XMPP (Extensible Messaging and Presence Protocol), formerly known as Jabber, is a secure, federated instant messaging system. Unlike Matrix, XMPP is designed to be lightweight and extensible through plugins, making self-hosting significantly easier with minimal requirements. Its open specification has led to adoption by government and non-profit entities for secure communications.

However, XMPP's extensibility results in fragmented user experience; features like audio/video calls may not be universally supported across all clients and servers. Encryption, while now standardized with OMEMO (comparable to Signal), was an optional extension. Despite these challenges, the author uses XMPP with Gajim and Conversations.im, finding them functional for text, audio, and video. Shufflecake's official chat is also hosted on XMPP.

The author ranks XMPP above Signal and Matrix due to Signal's centralization, Matrix's perceived centralization and "shady ecosystem," and XMPP's open nature, despite its UX inconsistencies and less robust cryptography compared to Signal. The article briefly mentions Simplex as a new alternative but expresses reservations after preliminary investigation. The author dismisses P2P IM solutions like Briar due to the requirement for simultaneous online presence.