User sucessfully migrated to the target. User is able to open the source mailbox using sidhistory, however they get the error:
"You are not allowed to send this message because you are trying to send on behalf of another sender without permission to do so. Please verify that you are sending on behalf of the correct sender, or ask your system administrator to help you get the required permission."
Sid History is sufficient to open mailboxes, however by Exchange design, is not sufficient to send mail as the user.
You must set the target account as an Associated External Account (AEA) on the source.
To do this, run a migration session on the users in question, with the options to disable the source account, and enable the Target account.
This will cause AEA to be set - the Source account's MSexchMasterAccountSID attribute will be populated with the sid of the Target account, and the user will be able to access their source mailbox normally.
For more information.See SOL22253 https://support.quest.com/Search/SolutionDetail.aspx?id=SOL22253