users in sub-domains

I have a single forest, mutliple domain structure with multiple sites.

Site 1 has a CAS/HUB, internet facing in domain main.local

Site 2 has a CAS/HUB/MB in domain main.local

Site 3 has a CAS/HUB/MB in sub1.main.local

creating mailboxes for users in wither main.local or sub1.main.local is not an issue.

Site 1 also has a subdomain called sub2.main.local.

I'm trying to create a mailbox for a user in sub2.main.local on the Site 2 MB server. I receive an error that the object cannot be found in my DC in Site 2. I can replaicate between all the DCs in all the sites. Looks like DC1 in main.local does not carry a copy of AD for sub2.main.local.