You guessed it right. No, it is not my first language that's why maybe I am more inclined to use paid tools like Grammarly. Although, my coworkers/employees are native speakers they make more spelling mistakes than non-native speakers (maybe confidence?). I force them to use grammarly and almost every email there is 5-10 corrections. If you didn't try yet, just download and scan some of your old emails. I bet you will find minimum of 5 mistakes. Better safe than sorry.