Not everbody use english as primery language, i have 5 languages in my reportuar, english as number 4..
I appreciate that you looking for an automated tool to make your posts more intelligible to other English readers, but, in my experience, the output of grammar checkers tend to be less intelligible. I find this especially true for technical writing (like on this board) where the grammar checker will often guess wrong about what part of speech some technical word is. This can lead to the content of the message getting garbled.
When I read a post with poor grammar I assume it to be a non-native English speaker, and read it a bit differently than a post with good grammar. When reading a post with proper grammar, but confused content, I assume the poster has no clue what they are talking about.
Often a person's ability to follow the more arcane rules of grammar (e.g. when to use "well" vs "good") is used as a proxy measure of intelligence. This may have once been reasonable, but in today's global internet culture it is clearly wrong.