[exim] Block if sender and recipient domain are the same?

Hi All

Is it possible to block an email if both the sender and recipient domain are the same?
Example - sender@???<mailto:sender@domain-a.com> to recipient@???<mailto:recipient@domain-a.com>

The reason I am looking to do this is we are going to be using Exim to handle incoming/outgoing mail for a domain hosted on Exchange so any mail for domain-a.com being emailed from a mailbox on domain-a.com will never leave exchange. We still want a sender on domain-a.com to be able to email external domains, but want to prevent incoming emails destined for domain-a.com appearing as if they are coming from domain-a.com.

The only alternative I can see at this stage is to have two Exim boxes, one to handle incoming the other to handle outgoing.

Thanks for any help.

Regards,

Pete.