UPN Suffix Routing - One Way Trust

Hi,

I am in the process of changing user UPNs to their primary SMTP attribute but have encountered a UPN Suffix routing issue on one of the forests. Details are:

Forest A:
DNS - ResourceA.Internal
Name Suffix Routing:
AccountB.Internal - *accountB.internal
AccountB.Internal - *accountB.com
AccountC.Internal - *accountC.internal
AccountC.Internal - *accountC.com

Forest B
DNS - AccountB.Internal
Alt UPN Suffix - AccountB.com

Forest C
DNS - AccountC.local
Alt UPN Suffix - AccountC.com

Trust Relation Ships
ResourceA.Internal <==> AccountB.Internal (Two-way - Forest - Transitive)
ResourceA.Internal ==> AccountC.Internal (One-way - Forest - Transitive)

ResourceA.Internal contains Mailboxes with disabled accounts and Servers
AccountB.Internal and AccountC.Internal contain user accounts linked to Mailboxes in ResourceA.Internal

AccountB.Internal behaviour:
I am able to access Mailboxes (via OWA) and RDP to servers with Domain\User and UPN.

Accountc.Internal behaviour:
I am able to access Mailboxes (via OWA) and RDP to servers with Domain\User only not UPN.

I have checked firewall ports and the following are open:
135/TCP      RPC Endpoint Mapper
464/TCP/UDP      Kerberos password change
49152-65535/TCP      RPC for LSA, SAM, Netlogon (*)
389/TCP/UDP      LDAP
636/TCP      LDAP SSL
3268/TCP      LDAP GC
3269/TCP      LDAP GC SSL
53/TCP/UDP      DNS
49152 -65535/TCP      FRS RPC (*)
88/TCP/UDP      Kerberos
445/TCP      SMB (**)
49152-65535/TCP      DFSR RPC (*)

How can I confirm DNS is working?
Any ideas what I can check next?