Follow up: Ran domainprep on the child domain.
Created the RUS for the child domain, had to make a couple of adjustments here for NETBIOS to resolve right. Still no luck with the OAB errors when the users hit Send / Receive to download them. I decided to try to setting up a test mail user in the child domain and see if the error continued. It took a few attempts and some (impatient) waiting for replication but I got my recipient policy set up to apply the correct email domain to the users in the child domain when I created a mail enabled user. Everything was looking good until I tried to access the new users mailbox via Outlook or OWA. I have found that while the Exchange attributes are look to be present in ADU&C, the mailbox does not appear in the mail store and the user is not in the GAL. The e-mail addresses are there, the exchange alias is there, and the record of the Mail Store being used is there, but there is no entry in the GAL, nor does the mailbox appear in the Mailboxes in the mail store in ESM. I even tried sending a message to the listed smtp address and got a NDR bounced back. Any thoughts?