Consolidating forests expertise needed

When using ADMT tool to migrate can any one tell me the end user experience. For example if I have two forests in two different locations. I choose to migrate users from forest A to forest B. Will the machines in forest A then be only logging onto forest B on start up. Will migrated forest A users still be able to access the same resources on forest A such as files etc.
And finally what happens their exchange email in forest A. Can they still access or will the mailbox now be inaccessible, thanks
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Sid_FAsked:
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Dariusz TykaICT Infrastructure Specialist Senior Commented:
When you migrate groups, users and computers to forest b users will be logging to forest b domain controllers.  Also during migration computers will be disjointed from domain/forest a and joined to forest b. So depending on your link speed latency it might be good idea to setup domain controller from domain b in location a as it may improve login process. Also during migration you have option to migrate users/groups SIDs to new domain (So called SID history). When user from new domain b will try to access resources from old domain a it will simply provide its SID from that domain and automatically granted or denied access based on previous configuration.
The whole process of preparing and executing migration using ADMT is well described in series of articles linked below:
https://blog.thesysadmins.co.uk/admt-series-1-preparing-active-directory.html
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Sid_FAuthor Commented:
Any details on what happens to exchange mailboxes if users are moved
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Dariusz TykaICT Infrastructure Specialist Senior Commented:
By default ADMT do not migrate exchange attributes so users will not be able to access Exchange. You would need to convert user mailboxes on Exchange to linked mailbox.
Here some more info on how to do that:
https://community.spiceworks.com/how_to/55657-how-to-migrate-users-and-computers-to-an-entirely-new-2008r2-ad-domain-forest-from-an-old-2003-2008-2008r2-domain-forest
IF CONTINUING TO USE OLD EXCHANGE SERVER IN SOURCE DOMAIN:

3. Repair email mailbox:

a. In Exchange Management Console of Source Domain, “Disable” User’s mailbox (Do NOT “Remove”, NOTE: This step will remove any non-default additional email addresses for the user. If this is a problem, you will need to notate what they are so that you can put them back in, OR, you will need to ignore this Step a, and Step b below, and instead use the Exchange Management Shell to link the mailboxes.

b. In Disconnected Mailbox window, Re-connect mailbox:

i. Select “Linked” mailbox, click next

ii. Select “Existing User”, and find disabled user in Source Domain, click next

iii. Select “Trusted Forest or Domain” and select NEW Domain

iv. Select the NEW Domain DC as the Linked Domain DC (using the local will eliminate the need to wait for AD Replication to occur)

v. Select the same username in the Target Domain as the Linked Master User, click next, then connect, then finish
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Windows Server 2012

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