It seems that SMA-X still sends e-mail to users that are inactive. In this situation we got LDAP users that get inactive because they leave the company which means they get removed from Active Directory but for history purposes get inactive in SMA-X. Because users get removed from AD they also won't have a valid e-mail address anymore. This is causing many e-mails to bounce as they can't be delivered. With a large enterprise company like us (15K users) we get 50.000 bounced e-mails in a month because of this.
To build: Don't send e-mail from SMA-X anymore to inactive users (from Administrator -> People -> Status: Inactive)