Send same inbound email to two separate Exchange email servers

I am in process of building a new Microsoft Exchange Server on a new domain.  I will be using Exmerge to move the mailboxes from the old server to the new server.  My plan is to shutdown the old mail server and to run Exmerge to create the pst files.  After running the second phase of Exmerge to upload the pst files to the new Exchange server I would like to setup a gateway SMTP host that will direct all new inbound mail to both Exchange Servers.  This allows me to the merge the old mail from the original Exchange with any new mail now coming into the new Exchange server.  I prefer this method as it allows me to gradually move over users from the old Exchange Server to the new Exchange Server.  Does anyone know of a method or third-party software that will allow inbound SMTP mail to be routed to two mail server whereby each mail server has the same copy of the email?

BTW the two Exchange servers are set-up with the same email addresses so internally both Exchange Servers will accept the same email.
dfkurzAsked:
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SembeeConnect With a Mentor Commented:
That type of migration has two options.

1. Big bang.
2. Planned co-existence. Email would have to go to the original server first and then get passed to the other server.
When I have to use that method I usually use dummy domains to get the email to move between the servers as quickly as possible.

Simon.
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SembeeCommented:
And your reason for using that method is?

You would be much better off doing a swing migration. Zero risk, zero downtime.

Also, you can't have email routed to two different servers. Once it has been delivered, that is it.

Simon.
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dfkurzAuthor Commented:
We have built a completely new AD domain/forest with the new Exchange Server and all new user accounts.  The old mail server is running Exchange 2000 on a completely different AD forest.  ExMerge will work to move the old mail but given the size of the stores and number of mailboxes it would be difficult to cut this all over at the same time while minimizing downtime.  I have the new Exchange server installed and all of the mailboxes created with the same email addresses as the old mail server.  

Inbound Internet mail is first passed to a inbound AV (Trend Micro) mail scrubber at our DMZ and then to IIS SMTP with GFI before using domain based policies to deliver the email to the inside Exchange server.  What I would like to do, in theory, is to have the IIS SMTP server first forward the email to the old Exchange Server and then a second copy of the email to the new Exchange Server.  In essence I have created a mirror of all inbound email.  I may be able to have Trend Micro do this task ... will investigate.        
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