I have often though this in work project emails to a much smaller audience (under 20 people). If I get one person replying back to me asking to clarify something, then I have failed in some way or another in my explanation, so I always read everything more than once.

I have noticed that many of our off-shore contractors, where English isn't their first language are prone to sending unclear emails - it's actually quite frustrating going through several replies to get to the bottom of it, and it often causes things to be delayed unnecessarily. If only everyone thought so carefully before sending the email!