Email Headers Show Non-Active Server

Hi- I'm doing some mail flow testing on a new deployment: single site, single domain, 1 CAS/MBX, 2 MBX, one DAG. I'm actually troubleshooting content filtering and came upon this:

I send an email from gmail to my Exchange account. The header includes this:

Received-SPF: Pass (MBX1.domain.local: domain ofusername@gmail.com designates 209.85.223.180 as permitted sender) receiver=MBX1.domain.local

The issue is that MBX1 is not doing anything right now but hosting a DB copy. The active/mounted DB is on the CAS/MBX server. So why does it say that MBX1.domain.local is the receiver???

Thanx

You should never, never doubt what nobody is sure about. -Willy Wonka