There’s an old adage, that what you say isn’t as important as how you say it. I disagree. Both what you say and how you say it are remembered.
If you write an e-mail with proper grammar, spelling and sentence structure, but use the BCC to talk about a person behind their back, you’ve done yourself a disservice. If you use poor spelling and sentence structure in your correspondence with your co-workers, how does your supervisor know that isn’t how you communicate with clients?
If you want to be perceived as professional make sure you communicate professionally. This goes for e-mail, in person, on the phone and every other way you communicate.