This question already has an answer here:
In e-mail correspondence, the author of an competing paper has admitted a shortcoming of their method, roughly: "Yes, our method X does not work when used with Y." This is not obvious from their paper and is quite a big point from my perspective.
Can I cite this e-mail correspondence in my master thesis/paper (is there a difference?) and use as motivation for my work? Is this impolite? If I just say, X does not work with Y without citation, I feel reviewers will not believe me/say I did not apply the method accordingly.