A method for communicating a message directed to or concerning a user (in which case the message may be a core service request), from a source entity (11) in a source system to a target entity (17) in a target system, the user having a first user identifier known to the source entity (11), and a second user identifier used by the target entity (17) to identify the user in the target system, with the second user identifier not known to the source entity (11), the method including: a step (71 81 82 83) in which the source entity (11) encapsulates the message in a first message envelope containing an element (52) including the first user identifier and an element (53) containing a user identifier placeholder, and also a link (L.sub.12) relating the first user identifier and the user identifier placeholder.