We're currently in the process of migrating from Domain X to Domain Y, starting with our User/Computer Accounts. Our Exchange and EV Servers will eventually be migrated, but for now they are in Domain X.
In testing I've come across an issue where once I've migrated a user account from X to Y everything on the Exchange front works fine, and all messages are available. When the user tries to open an Archived message they are prompted for their user details.... when they enter them in (DomainY\Username) the message opens without issue. Until they close Outlook they are no longer prompted, but if they close Outlook and reopen they are again prompted on the first Archived message.