The very nature of e-mail (which, along with first cousins IM and text messaging, is an undeniably handy means of chatting) encourages sloppy "penmanship," as it were. Its speed and informality sing a siren song of incompetent communication....it's not enough to simply vomit out of your fingers.
He is right, of course.
Writing well is hard work. Precisely written communications require not just writing, but editing, and rewriting. That takes time. Email's informality as well as it's perceived urgency contribute to the rush to get something out even if it is easily misunderstood. Most email senders don't have the time to be precise and admittedly, most email messages simply don't require precision. The senders assume that if the reader is confused they'll ask questions. If the sender is lucky, the reader can infer the message's intended meaning without conference. That's perfect for the sender. Sure, it's messy, but if the idea was to convey essential meaning at the least possible process cost to the sender, a poorly worded email was sufficient to the purpose.
Technology's speed of light evolution has put us in a rush to communicate as well. The technology idea marketplace teaches us that first-mover's have a tremendous advantage. Technology fairly screams "Be first to put our ideas into the marketplace! You can edit later!" This might be an artifact of the American obsession with copyright and trademark. If we can be first with an idea, we get to take credit for it. Alas, it provides even more encouragement for publishing incompletely formed, and sometimes slovenly communicated ideas.