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Exchange Routing Between Subdomains

Exchange Routing Question:

Current Environment:
I currently have a windows 2003 domain and an Exchange 2003 environment.  This environment is setup to receive email from a.com.  The MX records for this domain point to Microsoft Hosted Exchange which then routes the emails to our Exchange servers.  The internal DNS name for this organization is NOT a.com.

New Environment:
In order to support a different business unit within our organization we are setting up a parallel network.  That means a new Active Directory forest, new routers, switches, etc.  This also includes a new Exchange 2007 environment.  These two setups will be completely separate and they will not have any direct connectivity between them.  We are setting them up for SOCKS reasons and do not have a choice on the setup.  This is how it has to be.  The internal DNS name for this organization should be a.com.

Email Routing:
The idea, if we can make it work, is to setup both domains to be able to receive email with the same primary dns suffix.  What I mean by that is if I email a.com then the email gets delivered to the current Exchange Org.  If I email a subdomin (subdomain.a.com) then the email gets delivered to the new org.  This can be easily accomplished using subdomain MX records and Microsoft Hosted Exchange to route the emails to the correct external IPs of the 2 different orgs.

The problem I see in this is routing between the orgs.  If I am sitting in the new domain and my email address is  user@subdomain.a.com  and I email   user@a.com   the Exchange server is going to look inside its own organization for user@a.com, not find it and have issues.  The same will be true if I am sitting in the old org and try to email someone in the new org.

Now I am pretty sure that there is a way to get around this routing and to have the two orgs forward emails to the other org based on the subdomain.  What I am not sure of is how to do this.  Can anyone tell me?
1 Solution
The sub domain is the key here.
Put a sub domain on to each domain, even though the one on the primary will not be used outside.
The use SMTP Connectors (E2003) or Send Connectors (E2007) to route email for the sub domain directly. That will mean port 25 being open between the two networks, but you need that otherwise routing of the email will be a mess.

Then use mail enabled contacts for users in the other domain, with the primary address being the sub domain. On the E2003 server, add the main domain as well, so the contact has both addresses and and it will serve as the routing point for external email as well. http://www.amset.info/exchange/contacts.asp
Now I don't know if the email address forwarding trick works on E2007, I haven't tested it yet. If I get a chance I may do later in the week.


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