Here's our Exchange 2007 Scenario. (2 Exchange Servers on a different forest)
For ABC.COM Exchange domain 250 mailbox users, 300 mail contacts that we added to represent the other Exchange DEF.LOCAL domain users and 60 Distribution groups.
For DEF.LOCAL Exchange domain 200 mailbox users, 350 mail contacts that we added to represent the other Exchange ABC.COM domain users and 80 Distribution groups
The primary email address we are using that is configured in Exchange server is DEF.NET. Basically, for us ABC.COM users, we can send outgoing email and we can also receive incoming email as long as the email address is ABC.COM. But if someone wants to send an email to our DEF.NET email address, we need to ensure that the corresponding ABC.COM user mail contact is created in the DEF.LOCAL Exchange Server Mail Contacts section with an smtp address of user@DEF.NET.
In addition, in order for us to be able to send emails to users of DEF.LOCAL, we also need to ensure that their corresponding DEF.NET mail contact is created in the ABC.COM Exchange Server Mail Contacts section with an smtp address of user@DEF.net
This is also the same case of DEF.LOCAL Exchange users. Since their primary address is DEF.NET, they have no problem of sending and receiving emails from outside. But since the company requires us to use DEF.NET as the primary email address for both Exchange domains, in order for DEF.LOCAL users to be able to send emails to ABC.COM, the corresponding ABC.COM mail contact is created in the DEF.LOCAL Exchange Server Mail Contacts section with an smtp address of user@ABC.com.
By the way, both domains have domains trust in each other as well. The AD/DC OS on both forests are running Windows Server 2003 Standard. The version of the Exchange we have is Exchange 2007 SP3 installed on a Windows Server 2008 OS.
I am looking at GALsync program (http://www.msexchange.org/articles-tutorials/exchange-server-2003/migration-deployment/GAL-Sync-Identity-Integration-Feature-Pack-IIFP.html) but I am not sure if this will solve everything. Basically, since we're able to see users from the two Exchange domains with the use of Mail Contact, the objective now is for us to be able to see the corresponding distribution groups for both Exchange domain within Outlook GAL. As of now, the only way for us to be able to send email to the distribution group across the other Exchange domain is to create a corresponding mail contact of the particular distribution group. Although that works, it would sure be nice if we could also see the members of the distribution group as well from the other Exchange domain.
Thanks for taking the time to read this and thank you everyone in advance for the help and advice.