Really sorry about the sending error you're getting.
The VM501/453 error usually indicates too many recipients within an hour. But this shouldn't be triggered if you're only sending out to 10 recipients.
I suspect it may be something to do with Outlook's sending IP range which we should be able to update.
I've sent you a PM (purple envelope icon, top right of page) requesting the Header data from you. As soon as I have this I should be able to determine what we need to do to get this resolved.