This policy setting controls whether Outlook users can publish e-mail certificates to the Global Address List (GAL). If you enable this policy setting, the 'Publish to GAL' button does not display in the 'E-mail Security' section of the Trust Center. If you disable or do not configure this policy setting, Outlook users can publish their e-mail certificates to the GAL through the 'E-mail Security' section of the Trust Center.
Solution
Policy Path: Microsoft Outlook 2016\Security\Cryptography Policy Setting Name: Do not display 'Publish to GAL' button