Remove the check mark next to the field to allow users who are not mail enabled to be included in the Dynamic Group creation.
Navigate to the following location:
- Expand Configuration
- Expand Policies container
- Expand the Administration container
- Select the Builtin container
- Select Built-in-Policy - Dynamic Groups on the right window pane and double click to open
- Select the Policies tab
- Double click the policy showing in the screenshot below
- Select the View/Edit... button
- Select the Policy Settings tab
- Deselect the option Include only mail-enabled users in dynamic distribution groups, and select OK to confirm the changes