Mail bounces with DMN:MSG Recipient rejected: user@domain.com (4)

Mail sent to a valid GroupWise user bounces at the GWIA with a "recipient rejected" error, followed by (4).  The user is a valid user, and receives mail on a routine basis, except from this particular sender

Check Access Control Lists in the GWIA for any exceptions in the "allow" fields blocking particular domains, etc.  The (4) indicates that an access control violation of some sort has occurred.