Yes, you are allowed to lean over the lines with your body. The feet positioning is what is important. As long as – at the moment the ball is served – at least one foot is on the playing surface behind the baseline (4.A.2.a), neither foot is touching the court on or inside the baseline (4.A.2.b) and neither foot is touching outside the imaginary extensions of the sideline or centerline, then it will be considered a legal serve. Lean all you want.