As a security professional, you're probably already aware that, given time, even the most secure servers will decay into a dangerous state of disrepair. Clients are focused on their work, their products. Security tends to take a back seat once the client becomes more comfortable with such a stable email server.
Periodic status checks on the email security of the server are a must. Even a single missed hotfix or patch could leave the system open to attack, compromising all the information on the server. An opportunity may exist for your organization to provide periodic email security assessments on the system to ensure it's still secure. If the client has decided not to accept your offer of managed service for the system, this could provide a less expensive peace of mind for the client in the long run.