A Microsoft Exchange Server recipient that is not local and also not a distribution list or custom recipient is treated as a remote mailbox. Remote mailboxes must always have a valid O/R address (a distinguished name is optional) so that the X.400 backbones can transmit the message. If a distinguished name is routed successfully through the GWART and no O/R address is available, a new address is constructed from the remote-site-proxy O/R address information in the connected sites. If the new O/R address contains no DDA information, the distinguished name is saved in the DDA of the O/R address with the special "MSXCHNGE" DDA type to mark it as artificially constructed. This mechanism also enables encapsulation of Microsoft Exchange Server distinguished names over 1984 X.400 backbones.