Migrated Exchange users are unable to send to "restricted" groups in Domino
In Domino, "restricted groups" can be created in which only certain users are allowed to send an email to the group. When a Notes user is migrated to Exchange who was previously allowed to send email to the "restricted group" tries to send an email after migration, the email is rejected.
Coexistence Manager for Notes is not designed to circumvent Domino security. In this case, we would recommend contacting IBM for assistance in adding external users, to the list of users allowed to send mail to the Domino "restricted group".