Abstract: A way of synchronizing the contents of commonly-owned mailboxes (108, 111) in disparate messaging systems, such as an e-mail system (100) and a multimedia system (101), networked via the Internet (102). The e-mail system uses the existing auto-forwarding feature (115) of the Internet e-mail community to send a copy of every message arriving at the e-mail mailbox to the other system's synchronized mailbox. The other system applies a 3-part synchronization rule (113) to every message (200) arriving at its synchronized mailbox. If it detects a syncmark (204)--an identifier--of its own mailbox in the header (201) of the message, indicating that the message was previously copy-forwarded from this system, it just deletes the message.