As a businessperson, you're going to have to do some writing. Maybe not a lot, maybe not often. But at some point, you're going to have to put pen to paper - or rather, fingers to keyboard. Very little makes you look more amateur than simple errata. (Read that carefully. The last word isn't nearly as exciting as it looks on first glance.) So, from time to time, I'm going to give you some very basic writing tips.
The first one is this: SpellCheck. Use it. There isn't a word processing program is use now that doesn't have it. People don't make big mistakes, they make little ones. If you know you don't know how to spell a word, you look it up. But it's easy to get letters mixed up or omit a letter. How would it look if I had told you to us SpellCheck. But that's what I did the first time I wrote it. And how seriously is someone going to take you if you sign your letter, Sincerly?
SpellCheck has limits, of course. It didn't catch the error I just mentioned because us and use are both perfectly good words. SpellCheck will only tell you if what you typed is actually a word, it won't tell you if the word is the one you meant to use, or should use. But it will catch a lot of errors. When you write something like, The meeting is goign to be at 10:30, it will keep you from looking like a fool.
A lot of the mistakes it catches are just carelessness. Do you want to look careless to your clients and customers? And if you can't even be bothered to use a function that everyone knows you have and that takes almost no effort, why should your client think you're going to put much effort into her project?