When you woke up this morning, you realized that not only do you want to keep your job, but your boss's criticisms about your performance were right on target and easy to correct. You can stay a bit later at work. You can quit flitting around the office and gossiping with colleagues while your boss is out of town so that you don't miss his calls. You can slow down and use the spell and grammar checks before you hand work in. And yes, even your friends have questioned some of the outfits you've worn to work lately.
Words carved in stone or the modern day equivalent, email, are tough to take back.
Since your boss checks his emails from the road, chances are the message has already been read by the time you get to the office. Then add that you'd also like him to disregard your email. Whether or not you leave a voice mail, be prepared for negative fallout once your boss arrives in the office. And be honest. Tell your boss that you sent the email when you were too tired to think straight.