On the phone to Margaret today, she mentioned how she got back to her flat in Edinburgh after a couple of weeks in France, to receive, along with the rest of her email, a mailshot from [company]. Her comments to me were "It's so obvious this was written by a non-native English speaker. Did [boss] write this himself?"
Yes, [boss] did indeed write it himself, and didn't ask anyone to double-check it (I've occasionally bullied him into having people double-check previous newsletters, and they've ended up better written as a result, but I don't think he likes that). And it shows.
One thing she said, which I think is very true: it's a common mistake to think that because you can communicate easily in a language, that means you're fluent in it. It's one thing to have a funny accent and say things slightly strangely (or, as [boss] would say, "a little bit strange"); most of the time this doesn't matter, because you're dealing with existing colleagues, clients or suppliers, or, if you're trying to make a deal with someone who doesn't know you, they nonetheless know that you're the founder and boss of [company], a non-exec director of [other company], and in general someone to pay attention to.
When you're sending a mailshot out to hundreds and thousands of people, every little bit counts, and every single typo or weird phrasing counts against you. Consider that half of all CVs get rejected on sight because of spelling mistakes or grammatical errors, then apply that reasoning to what many people consider to be semi-spam in the first place... it's not pretty.
[Edit]: Apparently, [boss] did have the mailshot checked by native-English speakers. Then he added some more [boss]-ness. Feh.
[Further edit]: I've been asked to remove personal references from this post. As it was a comment about language, and not individual people, I think my point still stands.