If you could set something in an email to make it deliver more quickly, would you ever not set it? As other stated, there really is nothing you can do to improve the email delivery times.
Gateway mail daemons can induce further latency, and there is nothing you can about it. For example, smapd, the smtp gateway daemon in Firewall Toolkit, accepts emails and stores them in a sendmail spool area. Sendmail will then periodically scan the spool areas and pick up the emails and pass it on. How often sendmail scans the spool area is configurable, sometime it's 5 minutes, often it's more. So this adds several minutes to the latency. The spool area scanning time in standard RedHat is something like 30 minutes, but that only affects very few emails, since most emails will not be stored in the queue. I've seen Exchange systems have several hours in delays.
Also note that your scheme will probably cause more nuisance to innocent users. People will still enter fake or, worse, other peoples email adresses. Since you don't screen outgoing email anymore, innocent users will have their inboxes flooded by bogus emails from your domain. This might not be a big problem or concern. However, you should be aware of the problem.
