Exchange 2003 - 2010 InterOrg migration.

Hello Experts, can you please help-me in the following situation?

I am trying to understand better, a Exchange 2003 to Exchange 2010 InterOrg migration.
Let me give you my example:

Domain A.COM --> Exchange 2003
Domain B.COM --> Exchange 2010 (in fact I am finishing a 2003 to 2010 update on it, and is running fine...)

This two domains have different MX records each other, like, for example:
Domain A.COM --> MX.A.COM
Domain B.COM --> MX.B.COM

I am planning to move users (ADMT) and mailboxes in waves, from A.COM organization to B.COM organization, without changing the MX records, until the end of the migration.

These are my questions:

- I have talked with some people and googled about. I saw that, to do this, you need to create a "fake" accepted domain in the B.COM organization, like for example A.EXCH, after this I need to move the User/Mailbox and in the A.COM organization, create a contact for this "fake domain" in A.COM, directing the mail that arrives for the "true A.COM user" to this contact. Is this correct? Is this the only (best, easiest...) way to do this?

- If using this "fake domain strategy", I will have the user with a least two e-mail addresses (user@A.COM and user@A.EXCH...) in the B.COM organization. I will need to send mail to internet as @A.COM, in the B.COM organization. Do I need to configure some option in Exchange 2010 to be able to send a mail as @A.COM, or i just need to set the user@A.COM as a default smtp address for this user and everything will be fine?

I kown that, for correctly being accepted from internet MX servers, I need to configure this B.COM server in "SPF"...

- After moving all User/Mailboxes, I just have to change the DNS/MX and configure A.COM as a Authoritative Accepted Domain in the B.COM organization, correct?

- What is the command (if you have examples, I would be pleased about...) used in powershell to move mailboxes Inter Organization?

Thanks for the help guys!
Who is Participating?
you only need to create the contact if you want the name to appear in both address lists

for mail flow purposes you need to update the recipient policy for the domain and remove the check that makes it authoritative
then you need to create an smtp connector for that domain that sends to a smart host (your new org)
thiagotietzeAuthor Commented:
So, the point about recipient policies is that I don't want to give all my users the @A.COM suffix. So I am planning to give this only to the migrated users.

So, for sending mail through B.COM organization to the internet, I need to register the domain as not authoritative?
thiagotietzeAuthor Commented:
In fact I have not been answered in all my questions, but I could move forward with the information provided.
I already have other doubts that will ask in another question, even regarding the Exchange InterOrg migration.

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