Originally Posted by KrisH

yes, and so does the exchange server.

I wont go in to details about the setup for obvious reasons.
The mail lands on MX A (smtp) and relays it to MX B wich is my exchange server.

MX B Only accept incomming traffic from MX A.

That keeps the ddos attacks and scriptkiddies away

When sending email from my Outlook or outlook-webaccess, it is send out by MX B (domain.local)