The real problem with grammatical errors is that regardless of whether or not the person can understand you, you're decreasing their level of processing fluency. High processing fluency has been shown to be linked with positive affect [1]. This means that by writing with grammatical errors, you are literally making the reader feel less positively about you and what you're saying.
Daniel Kahneman refers to this as "cognitive ease" vs. "cognitive strain". Making your writing correct and easy to read increases cognitive ease, which makes your readers "like what [they] see, believe what [they] hear, trust [their] intuitions and feel that the current situation is comfortably familiar." [2]
Grammar is important for making a good impression as well, but that's really not the key selling point, especially for anyone in business. The key point is that if you want to persuade anyone of anything, whether it's to invest in your startup, to buy your product, or to leave their cushy Google job to work for you, you will be more persuasive if you use proper grammar.
Daniel Kahneman refers to this as "cognitive ease" vs. "cognitive strain". Making your writing correct and easy to read increases cognitive ease, which makes your readers "like what [they] see, believe what [they] hear, trust [their] intuitions and feel that the current situation is comfortably familiar." [2]
Grammar is important for making a good impression as well, but that's really not the key selling point, especially for anyone in business. The key point is that if you want to persuade anyone of anything, whether it's to invest in your startup, to buy your product, or to leave their cushy Google job to work for you, you will be more persuasive if you use proper grammar.
[1] "Effects of Perceptual Fluency on Affective Judgments" http://psy2.ucsd.edu/~pwinkiel/reber-winkielman-schwarz-Flue...
[2] Thinking Fast and Slow