Yes but the opt-out was added only after they got banned

at the amount of grammar mistakes

Native speakers don’t usually make major grammar mistakes. They may not follow prescriptive rules, but they’re generally understandable by other native speakers because grammar is so deeply embedded in their subconscious that they can’t help handling the language correctly. You do the same in your native language. Everyone does.

The problem with non-natives, and I include myself as a non-native speaker of a few languages, is that we don’t usually have the same instincts. It would be pretty arrogant to tell a native that they don’t know how to use their own language when we, almost by definition, cannot possibly understand it in the same way that they do.

Email spam usually has heavily flawed English.

I've heard that this is intentional. It would be a waste of the spammer's time to be contacted by people who are smart enough to not be fooled. Those smart people won't bother contacting the spammer and wasting the spammer's time if they see grammatical errors in a message that purports to be from a reputable organization, so the spammer throws in some errors to make the smart people filter themselves out. Or so the theory goes.

*nitpicker (but I prefer pedant in polite circles, and grammar nazi on the Internet, or at least I did until actual nazis started showing up again)

Certain uni composition students had better learn to write flawless English if they expect to earn their desired grade in my courses.