I have site A with the domain blah.local, this site has Exchange 2013 installed. I have a remote site B with the domain blahblah.local both of these domains are 2012 function level. Site A is the parent company of site B and would like to "host" the email for site B. I want site B's Exchange email to use the same password they use when logging into their workstations at site B. No other access is needed between the two sites. I can create a VPN tunnel between the two sites. Is a transitive 1-way trust the best way to set this up? Once I setup the trust then can create a mailbox linked to the AD account in the domain blahblah.local at site B? As I mentioned I really only need the passwords sync'ed and I want to keep this as secure as possible. Any thoughts or ideas or better way to do this?

Hello.
You need to use the FIM 2010 R2 (only support Exchange 2013 and Windows 2012) for synchronizing accounts in t

Hi damadhatter,

Thank you for your question.

Is a transitive 1-way trust the best way to set this up? Once I setup the trust then can create a mailbox linked to the AD account in the domain blahblah.local at site B?

A: yes, we need to setup the trust on two site for link mailbox and FIM.

The synchronization service was included on Forefront Identity Manager(FIM),  we could refer to the following link:

http://blogs.technet.com/b/server-cloud/archive/2012/06/14/forefront-identity-manager-2010-r2-now-available.aspx

The more details could be referred by the following link:

https://technet.microsoft.com/en-us/library/jj134310(v=ws.10).aspx

If there are any questions regarding this issue, please be free to let me know. 

Best Regard,

Jim

So I have to use FIM to accomplish this? A simple trust would not work? I don't want to have accounts in both domains, I was hoping to attach the mailbox to the users account in site B under the domain blahblah.local  is that not possible? 

Pretty sure I found the answer to my question. Linked mailboxes, no need for FIM.

https://technet.microsoft.com/en-us/library/jj673532(v=exchg.150).aspx

Hi ,

If this link could meet your requirement, we will make as answer and close this thread.

Right?

If there are any questions regarding this issue, please be free to let me know. 

Best Regard,

Jim