Exchange 2010 - external domain trust

Hello All;

I'm kidding banging my head with this one and need some insight.  There is a two way trust between DomainA & DomainB

DomainA
Exchange 2010
server 2008

DomainB
server 2012R2

We have external users on DomainB that was primarily used for sharepoint authentication through the domain trusts.  
Now there is a SMTP requirement to have users in DomainB be notified for custom sharepoint tasks.
I wanted to create Mail Users & add existing users within Exchange 2010 for those users but Exchange only sees the Organizational Unit for DomainA.  

Domain A
Exchange 2010


Domain B
USERS (Need to Add Existing Users SMTP addresses).

Hope I didn't lose you and thank you for any insight.
synertiaAsked:
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Will SzymkowskiSenior Solution ArchitectCommented:
I think you are getting confused here (possibly). Mail User is an Active Directory Account that has an external email address. This AD account can access resources in the domain but the user does not have a mailbox within Exchange in that down. All Email will get sent to the external smtp address of the mail user.

As you have stated Domain B does not have an Exchange environment. I think what you should be referring to is "Linked Mailbox"

This is a mailbox that is in an Exchange environment (domain A) but is being accessed with an Active Directory Account that is in another Forest.

Try looking at Linked Mailbox as you need to have a Forest Trust in place to accomplish this.

Will.
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