Ms. Calendar received the following (edited slightly for clarity):
When setting up an impersonation account in our active environment the first few steps (editor's note: see The Cookbook Version of Exchange Migration Rights) go fine and then when I run this:
Get-Mailbox -resultsize unlimited | add-mailboxpermission -user OurDomainUser -accessrights: fullaccess -InheritanceType: All
It just sits there and never spits out any output. Yesterday I left it running for hours and eventually Exchange Management Shell closed itself but I still get the you don't have access to this email when trying to log in to any email accounts with the service account.
An interesting case which does occur!
There is not a lot of love across the community for Get-Mailbox performance and even less for practical actions to take. See: Performance issues with Get-Mailbox -Database PowerShell cmdlet and Count Mailboxes Per Database Faster.
We have seen in instances where the performance is deplorable it's usually a problem getting the "join"s to work through AD. Keep in mind we're linking two separate AD actions here, Get-Mailbox and Add-MailboxPermission.
On the related issue when you have trouble with Exchange performance itself, see Troubleshooting performance issues with Exchange when RPC request spike high.