Use this parameter to grant the trusted user Send As permissions to the mailbox specified by the Identity parameter. This ...

Use this parameter to grant the trusted user Send As permissions to the mailbox specified by the Identity parameter. This parameter accepts the arguments True or False. These arguments are not case sensitive. When using a CSV file, specify GrantSendAs True to grant the permission or GrantSendAs False to not grant the permission. Specifying False will not remove the permission if it is already set. To remove the permission, use Remove-MSOnlineMailPermission.