> I may use Delivery-By: if it's time-critical ("Hey George,
> lunch at Blue River
> Cafe at 12:30?") and can be discarded if the time constraint
> isn't met. In this case, I don't care about order, I just care if *THIS*
> mail gets there by a specific time or not.
Is "Delivery-By" a commonly used header, or just something
you suggested? I am sure the fundamentalists will say that
this should be on the envelope and not in the heading!
Deliver-by isn't a header, it is an SMTP extension. See RFC 2852.