Cannot respond to external Notes invitations

When users receive meeting invitations from other companies that use Notes, the reply address for the chair is their internal Notes name, i.e. John Doe/CompanyX, even though CompanyX is not associated with our company. So the user cannot respond to the invitation because of course the user name is unknown. I have searched the forum and others have experienced similar problems, but so far I haven’t seen a solution. We could respond to invitations properly when we used Outlook, and now the users want to switch back. Please advise on how to fix this. Thank you.