Exchange 2007 and 2010 Cross Forest Mail-Flow

Hi

We have a scenario where we have an Exchange 2007 organization (Org A - Source) and a separate Exchange 2010 organization (Org B - Target). Both the organizations (AD Forests) have two way trusts between them. We want to have our Exchange 2007 users have their mailboxes hosted on Exchange 2010 organization. Meaning that the user (AD) accounts of Org A will remain in Forest A but there mailboxes will be available on Exchange 2010 servers in Org B. So we don’t need a user / group migration stuff here and mailbox data on Exchange 2007 servers is also not critical (not required to be migrated) so we will be creating new disabled accounts and mailboxes in Org B for each user in Org A and connect the mailboxes. There will be a brief period of co-existence between two Orgs (to enable mail-flow) until all the users have their mailboxes active in Org B.

Currently, there's only single point of entry and exit on Internet for both the Exchange organizations. there's a IRON Port appliance that is pointed by MX records for bot ORgs. A rule on IRON Port relays e-mail for Org A to HT in ORg A and mail fpt Org B to HT in Org B. Outgoing e-mails from HTs are sent to IRON port (smart host) and then further relayed on Internet. There are SMTP connectors between the organizations to enable direct e-mail flow currently.  

To be able to setup e-mail flow during new mailbox creation, we are planning to follow the steps below:

Source Organization:

1. Backup and delete the mailbox from source
2. Create a contact that and stamp it with @source.com (Primary SMTP) and @target.local (External Address - internal routing domain).
3. Create a send connector for @target.local and point it to HT in target organization
4. Change the source.com accepted domain to Internal Relay instead of Authoritative

Target Organization:

1. Create a new accepted Authoritative entries for @target.local and @source.com (set as internal relay in source organization)
2. Create new EAP (or change default - please suggest ?) to stamp @source.com and target.local on new mailboxes to be created. Limit it to stamp only users in parpticular OU
3. Create new mailboxes (representing mailboxes from source organization) and verify they are stamped with @source.com and target.local e-mail addresses. 

Although everything else seems to work except the fact that now the source.com SMTP is defined as Authoritative domain in target. Our doubt is that when a internal user in target.com tries to send e-mail to a user with mailbox still residing in source.com organization - will it get delivered (we have no contact in target forest to represent mailboxes that still reside in source organization)? or will it get a NDR. (As I said earlier currently there's a direct e-mail flow between two organizations - will this work after making above changes ?).

If the mail-flow will not work, what could be the other strategy / change that we can make here to achieve our goal?

Don't know - can we get through this by defining source.com as Internal Relay in target as well (where we defined it as Authoritative)? Will this work?

Thanks

Taranjeet Singh

zamn