An interworking function is provided between first and second messaging domains or technologies, when original address/identities of users in the first domain and the second domain are incompatible. Upon receiving from a sender in the first messaging domain a message addressed to a recipient in the second messaging domain, the interworking function provides, for an original sender address or identity received in the message, a new sender address which is valid in the second messaging domain, and inserts the new address in a header field of the message to be forwarded to the recipient in the second messaging domain. Moreover, the interworking function adds the original sender address into the specific header field of the message either as a part of the new sender address or as an attribute.