For on-prem, you can leverage S/MIME:
https://learn.microsoft.com/en-us/exchange/configure-s-mime-settings-for-outlook-web-app
OME exists in Exchange Online Only:
https://learn.microsoft.com/en-us/microsoft-365/compliance/ome-faq?view=o365-worldwide
If you are using hybrid, you can enable a mail flow rule to encrypt the message however:
https://learn.microsoft.com/en-us/microsoft-365/compliance/ome-faq?view=o365-worldwide#my-organization-has-an-exchange-hybrid-deployment-can-i-use-this-feature