Hi Richard,
Do you mean you want to dismiss Exchange 2010 in Fabrikam.com and host all mailboxes in Contoso.com? Additionally, users in Fabrikam.com can still use their original credential (user@fabrikam.com) to
access mailboxes hosted in Contoso.com. Please confirm whether the following Exchange resource forest topology can meet your requirement:
https://technet.microsoft.com/en-us/library/aa998031(v=exchg.150).aspx
If that is the case, please try the following steps:
1. Move all mailboxes from Fabrikam.com to Contoso.com.
New-MoveRequest -Identity 'user1@fabrikam.com' -Remote -RemoteTargetDatabase DB03 -RemoteHostName 'CAS01.fabrikam.com' -RemoteCredential $Cred -TargetDeliveryDomain 'mail.contoso.com'
https://technet.microsoft.com/en-us/library/Ee633491(v=EXCHG.150).aspx
2. Convert migrated mailbox from regular user mailbox to linked mailbox:
Disable-Mailbox -Identity User1
$cred = Get-Credential
Connect-Mailbox -Identity User1 -Database "Mailbox Database" -LinkedDomainController FabrikamDC01 -LinkedMasterAccount user1@fabrikam.com -LinkedCredential $cred
https://technet.microsoft.com/en-us/library/Bb201694(v=EXCHG.141).aspx
3. Create accept domain for Fabrikam.com.
4. Create email address policy for with accept domain Fabrikam.com and apply this policy to linked mailbox.
This is only mu personal suggestion and hope anyone else can provide more ideas for your reference.
Regards,