The Delivery tab has a check box "Offer TLS client certificate" where a certificate can be selected. Change to a different certificate or uncheck the box and click "Save".
(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.)
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