[Solved] Office 365 : Block sending external e-mail, but allow internal e-mail

Here you find a powershell script which was very useful for me.
All scripts are free of charge, use them at your own risk :

This week I had to block some internal users, from sending external email.

They were allowed to send internal e-mail but no external e-mails were allowed.

Place all of these users in a single group.

Exchange => Mail flow => ADD ‘Restrict messages by sender or recipient…’

Select if ‘The sender is a member of’ AND select the group or persons who are not allowed to send external e-mail

Add a second action:

If ‘the recipient is located’ AND select Outside the organization

Do the following:

‘Reject the message with the explanation…’ And Type your preferred message.

Place the rule on top of the other rules, by using the Up Arrow