"Email address no longer valid" mail tip pops up when you add a migrated Exchange user to the recipients of a new email.br/>
The user is migrated a short time agobr/>
If the problem is related to synchronizations, then just update the global address list. + change the OAB polling interval temporarely on the CAS server(s) so that the OAB is copied from the OAB generating mailbox servers to the CAS servers OAB virtual directorybr/>