PHP Mail Configuration

Hello all!

I am fairly new to the administrative side of cPanel/WHM and could use some help understanding how (and if it's even possible) to do what I would like.

I want to to disable PHP scripts (scripts using the mail() function and SMTP if possible) from sending messages externally, but still be able to send messages to internal email addresses. I thought I had read about this somewhere before, but decided I didn't need it at the time and have since been unable to relocate the place I read it.

Email is being handled on the same server as the web hosting.

I need the following to remain functional:

• Email addresses need to be able to send and receive internally and externally.
• PHP using the mail() function or SMTP need to be able to send internally to existing email addresses (or to forwards)
• Some accounts are using dedicated IP addresses, so I will need to make sure they are able to send from their scripts internally as well

I would greatly appreciate any assistance with this.