Everyone knows that grammar is important, especially in professional correspondence. If you don't want your colleagues to read your emails in a Cletus the Slack-Jawed Yokel voice, you need to make sure they're written in proper English.
The problem, of course, is that many of us don't know what proper English is. As Alison Griswold pointed out on Forbes.com last year, our grip on the language has slipped, to the point where many of us no longer use the subjunctive (or know what it is) and frequently mix up homophones like there/they're/their. Assuming we don't have time to retake freshman English, what are we to do?