Some users in my organization are not permitted to send external e-mail. How do I enforce this.

Detail

You can do this using the relay settings. Select the Relaying page in the Settings property sheet. Select Allow specific relaying and press the Settings button. In the Sender tab of the Relay Settings property sheet, select Permit all except specified addresses radio button and enter the e-mail address of the user who you wish to prevent from sending externally into the field at the bottom of the property sheet and press the Add button. Press OK to save you changes.