As is questioned above, daily English seems not enough for work. Plz share some thoughts.
I consider that there are two parts to professional business writing: correctness, and style.
The first part is correctness. Professional business writing should use correct spelling and grammar. (That may seem obvious, but it's frequently not the case.) The writing should not use SMS-style words, and should not use slang.
The second part is style. If your writing is 100% correct, it may sound terse or robotic. That is not a good way to connect with partners and customers. It is important then to allow your own personality to enter your writing. Don't use slang, but do be friendly.
The only other point I would make is that, beyond correct spelling and grammar, you should not "try" to sound more professional. This may cause you to use longer words or more complex sentences. While it may feel more professional to you as the writer, it may seem phoney to the reader. The point of any writing is to communicate with your reader, so clear, simple, and friendly language is usually best.
If you're truly keen to improve your writing, I recommend highly the book, "The Elements of Style" by William Strunk.