1. A computer readable medium including program code for providing a message-based interface for exchanging information about email activity, the medium comprising: program code for receiving via a message-based interface exposing at least one service as defined in a service registry and from a heterogeneous application executing in an environment of computer systems providing message-based services, a first message for maintaining information for one or more email activities, the maintaining using email activity data obtained from imported structured data, the first message including a message package hierarchically organized as: an email activity bundle maintain request sync message entity; and an email activity package including at least one email activity entity, wherein each email activity entity includes a message from party from a party package, and wherein each email activity entity further includes at least one of a reference party entity, at least one message to party entity, and at least one copy message to party entity; and program code for sending a second message to the heterogeneous application responsive to the first message.