mailflow in a mixed environmnet (Exchnage server 2003 and Exchange server 2007)

how can i identify the exact mailflow, still we are using exchange  server 2003 and exchnage server 2007.
We are using Spam and virus firewall using Barracuda ,  when i checked in the barracuda firewall ,
i could find ount that the destination mailbox server  TCp/ip deatils , still it is pointing to-
Exchange server 2003 Bridge head server , so this means when external user send mails to my -
organization (abc.com) , the mail fisrst comes into BARRACUDA Firewall and  then point to Exchnage server 2003
then the exchange server 2003  redirect this to hub transport server of exchnage server 2007 . once the mails comec into hub -
transport server , the hub transport server point to correspond mailbox servers for the maildelivery,
  Is there any method for finding the mailfolw ? how can i ensure that  still the mailflow happens through the Exchange server 2003 (Bridge Head server )
????  Let me know any tools availble for findintg the mailflow ??? any tricks avialble ????
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