That's a trial and error process of proof reading your writing. Inefficient IMHO. A solid approach would be clarity of writing through lucid and cogent thought; if your thinking is clouded so is your writing.
It never hurts to read aloud your email to be sure it "sounds" right but that is, I assume, something everyone practices before they post, email, or submit.
That's basically using an audience of 1. Starting with yourself and checking if it sounds right. After that, you'd probably bounce it to a few people in the office, see what they think. They might pick up on something that sounds clear to you, but doesn't make sense to other people.
Finally, you would want to bounce it to a few customers. Everyone inside the company is going to have a similar set of knowledge and assumptions when they look at the email, the only way to really be sure is to send it out.
I don't think he's saying you should write down some random thoughts and test them, but rather as a final proof-reading step before sending it out to everyone.
Even lucid, cogent thought can result in confusing prose. Proofreading your own writing is necessary, but not sufficient.
The problem is that, for you, the abstract thoughts in your head came first, and the prose came second. When you re-read your prose, you're reminded about all of the implications of the abstract thoughts in your head. When someone else reads your text for the first time, they have to construct the abstract thoughts from your words alone. This process reveals confusion that you probably would not find on your own.
"but that is, I assume, something everyone practices before they post, email, or submit" I like your optimism, but I'm guessing the majority of people do not in fact read things aloud before submitting. I'm sure it's different when sending an email to thousands of customers, but the quality of comments on any forum site suggests it's nowhere near universal.
It never hurts to read aloud your email to be sure it "sounds" right but that is, I assume, something everyone practices before they post, email, or submit.