This point has been made before, but every now and again, it seems wise to bring it up again. Spell check programs will tell you if a word is spelled incorrectly … in its limited awareness.
Mostly, what this means is that regardless of how it is spelled, the program does not know if that’s the right word to use. It doesn’t know if you should use affect or effect, for instance. Or their or there. And the Internet is rife with examples of mistakes it thinks are just peachy-keen. “Marketing are bad for brand big and small. You Know What I Am Saying?”*
Also, these programs have extensive lists of words that will cover what most writers will use. But if your vocabulary is more extensive, it may tag words as misspelled simply because it does not know the word. So, clearly, spellcheckers alone are not enough.
That said, at least do that much! If you want to present yourself as a professional and you misspell common words, no one is going to take you seriously. They’re going to think you’re stupid. Or at the very least, incredibly lazy. I’m not saying you are, I’m just saying that’s how you will look to others in business.
So, please, use a spellcheck program, but then have someone look it over for you, too. And please do not attempt to proofread your own writing. No matter how objective you think you can be, you can’t. My favorite example of that was when I missed that I had misspelled proofreader on my resume. No, I am not kidding. No wonder I didn’t get a lot of response with that resumé!