Many have asked about non-native, but what about Americans born and raised here that work in a professional environment working with clients on a daily basis?
I believe that when talking to clients it's important to have a strong foundation. You should know how to use seen/saw, your/you're, there/their/they're just to name a few examples.
When I see emails with a list of CC'd clients and we're not using the proper English it makes us look uneducated and unprofessional. Am I overcritical and if I'm not how do I approach this?
Yes, I know mine isn't perfect either. I personally don't take offense when someone does point out that I'm abusing the comma, ending a sentence in a preposition, or using the wrong spelling of 'principal' for example. In fact, I'm thankful. It's partially how I've gotten much better with it and maintain it after not having been in school for years.