Add-Type -AssemblyName Microsoft.Office.Interop.Outlook

$folders = "Microsoft.Office.Interop.Outlook.OlDefaultFolders" -as [type]
$outlook = New-Object -ComObject Outlook.Application
$mapi = $outlook.GetNameSpace("mapi")
$tasks = $mapi.getDefaultFolder($folders::olFolderTasks)

$currentTime = Get-Date

foreach ($task in $tasks.Items) {
    if ($task.DueDate -lt $currentTime) {
        $task.Complete = true
        $task.Save()
    }
}