So I'm trying to clean up permissions on the mailbox of one of our former employee, John Smith. There's an SID listed as having "Send As" permissions to his mailbox that I'm trying to remove. The command should be:
Remove-ADPermission -Identity "email@example.com" -User "S-0-0-00-0000000000-000000000000000000-0000" -ExtendedRights "Send As"
But that gives me a " 'firstname.lastname@example.org' wasn't found" error, as does any other variant of his identity (jsmith, 'smith, john', etc.). His mailbox isn't hidden. Any suggestions?