How to remove an expired certificate from Symantec Messaging Gateway
The certificate <example certificate name> is currently in use by the (inbound | outbound | delivery) MTA on host <example host>
You can delete certificates in the Control Center
(Administration -> Certificates)
Make sure all the places where a TLS certificate can be selected have been disabled.
(NOTE: Old certificates can be selected on tabs that are not currently visible in the Control Center. Enabling "Inbound and outbound mail filtering" temporarily will allow you to see all tabs and select a different cert. After saving the settings, you can return the setting to your preferred configuration and save the settings again.)