"I'm certainly getting a lot more mail... that's basically it"

Sam Mendes is most likely referring to the increased attention he has been receiving due to his success in the movie industry. He has likely been getting a lot of fan mail, as well as deals for new tasks and cooperations. He is most likely overwhelmed by the amount of mail he is getting, but is likewise grateful for the recognition and chances that include it. He is likely humbled by the attention and is taking it all in stride. He is most likely content with the fact that he is getting a great deal of mail and is not expecting anything more than that. He is most likely appreciative of the recognition and is taking it as a sign of his success.