Dirsync runs but objects are not getting mail and mailbox enabled, when reviewing failed objects DSA complains that it cannot copy the "msExchUseOAB" attribute.
Source "msExchUseOAB" attribute has a value which cannot be applied in target.
In Active Directory the msExchUseOAB attribute in the msExchPrivateMDB class can be configured to use a specific offline address book for a particular mailbox store on a database. The msExchPrivateMDB class determines the configuration on a private database. The value of the msExchUseOAB attribute determines the specific offline address book that a particular mailbox store uses and is the distinguished name the offline address list the users will download.
To copy this value to the new domain doesn't make any sense, it cannot be used there anyway.
Skip this attribute (at least for the object class "Users").
After skipping it from sync (skipping two-ways) accounts should get mail enabled immediately.
How to verify which (and if) an offline address book is associated with a mailbox store you can use this procedure (you can also use it to associate the Offline Address Book with a particular mailbox store in Exchange Server 2003)
Start the Exchange System Manager.
Expand the server and the storage group affected.
Right-click the mailbox store and click Properties
Click the General tab, and then click the Browse button that is on the right side of the Offline Address Book box.
Enter an offline Address Book to associate with this mailbox store, and then click OK
To associate the Offline Address Book with a particular mailbox store in Exchange 2007
In the Exchange Management Console, expand Server Configuration, and then click Mailbox
In the Database Management window, right-click Properties on the mailbox store.
Click the Client Settings tab, click Browse, and then click the appropriate offline address book.
Click OK two times to finish.