If all you truly have is "Outlook Anywhere" access, then I'd presume that you don't have access to administer the Exchange Org in Forest A.
If that's the case, then you'll need to manually migrate, which won't be fun with a lot of mailboxes.
You'll also be dealing with issues related to if you have public folders, GAL issues when it comes to replies to existing emails, certificate issues (possibly), and the list goes on.
Exporting/Importing the existing mail is the easy part actually here.
You can simply create a local PST file for the mailbox, move (or use the AutoArchive function and set it to 1 day) the email, calendar, contacts to the PST file. I would also HIGHLY suggest moving the SUGGESTED CONTACTS folder as well, since Exchange 2010 stores the autocomplete addressing now, and you won't have that without a true mailbox move.
Don't forget about resource mailboxes, shared mailboxes, etc.
Like I said above though, I hope this is a small organization and that email isn't a critical medium for them. You need to plan out the migration way beyond looking at just email.
IF you need to bulk export my recommendation would be to contact the admins of the forest the mailboxes are currently in and have them do bulk exports via Exchange Powershell (export-mailbox
) for all of your mailboxes. NOW...this isn't exactly a fast thing with 3,000+ mailboxes, and it is only a point in time snapshot, so you'll want to migrate ALL the users FIRST to your new mail system, and then have the old forest Exchange admins do this bulk export and send you the mailbox PST files. You can then look at importing the PST files back into the users new mailboxes on the new Exchange org. BUT, be absolutely sure with this many users that you approach this right, especially if you think this going wrong could be disastrous for you and the company.