How to prevent sending external e-mail, but allow internal e-mail in Microsoft Exchange Server.

Sometime may be you have internal & external domains configured in your organization and you don't want allow internal users to send emails outside the organization or to the public domains. You can achieve this using Microsoft Exchange Transport rules.

Suppose you have all internal users in OU name "InternalMail" create a distribution group in the OU and add all internal users as member.

Open up Exchange admin console "https://yourexchangeserver/ecp" and navigate to "mail flow" -> "Rules"

Click the + button -> "Restrict messages by sender or recipient"

In this example "extblk" is a distribution group. Added two conditions first is "the recipient is located" second is "the sender is a member of"

Review the Rule mode is set to "Enforce" and leave other settings default and save the rule.

You are done !