User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Preferences > Search Options
User
HKCU\software\policies\microsoft\windows\windows search\preferences
This policy setting allows you to turn off the indexing of the body and attachments of clear-text signed messages. The sender, subject line, and date will continue to be indexed and searchable. If you enable this policy setting, indexing of clear-text signed messages will be turned off. If you disable or do not configure this policy setting, clear-text signed messages will be indexed and searchable.