The Matrix client-server and server-server APIs are largely expressed in Matrix user identifiers. From time to time, it is useful to refer to users by other (“third-party”) identifiers, or “3PID"s, e.g. their email address or phone number. This Identity Service Specification describes how mappings between third-party identifiers and Matrix user identifiers can be established, validated, and used. This description technically may apply to any 3PID, but in practice has only been applied s...
