Rule to get 365 to reject email at the SMTP stage

Question

I have setup a shared mailbox to use for sending alerts from a powershell script and it works fine.

I now want to lock down the shared mailbox so that it cannot be used to send outside the org, and so I followed the instructions at,

https://www.codetwo.com/admins-blog/how-to-prevent-office-365-users-from-sending-emails-outside-organization/

and it works except that 365 accepts the message and it then leaves a rejection notification in the inbox of said shared mailbox.

Is there a way to drop the message at the smtp connection stage so the inbox doesnt get littered with these rejection notifications? Not a big issue, it just seems a cleaner way to do it.

Cheers.