Is it possible to add digital signature to outgoing messages for all exchange users? Currently we have add digital signature individually using Outlook.

Update to my question:

Can we use our internal CA to publish certificates.  We only want digital signed emails for internal users.

Add Digital Signature to outgoing messages is a feature of Outlook client:

https://support.office.com/en-au/article/Secure-messages-with-a-digital-signature-549ca2f1-a68f-4366-85fa-b3f4b5856fc6

It can't be configured in Exchange server.

Hi Monolithickernal,

Transport Rules don't support S\MIME, it works with IRM only.

Best you can have is a look into below features:

1. Outlook Protection Rules

In Exchange 2013, Outlook protection rules help your organization protect against the risk of information leakage by automatically applying IRM-protection to messages in Exchange 2013. Messages are IRM-protected before they leave the Outlook client.

2. Transport Rules

Transport protection rules allow you to use transport rules to IRM-protect messages by applying an Active Directory Rights Management Services (AD RMS) rights policy template.

You can also try implementing Tri_angle's recommendation using a GPO.

You can customize these security settings for users in advance. You can use registry settings or Group Policy settings to customize Outlook to meet your organization's cryptographic policies and to configure (and enforce, by using Group Policy) the settings that you want in the security profiles. These settings are described in Configuring Outlook 2010 cryptographic settings later in this article.