Apply sin by mail

I think there may be a bit of a misunderstanding here!

"Apply sin" is not a valid phrase, and I'm assuming you meant to ask about applying the sine function, which is a mathematical operation.

If you meant to ask about applying the sine function, I'd be happy to help. The sine function, denoted by sin(x), is a trigonometric function that maps an angle to the ratio of the opposite side to the hypotenuse of a right triangle.

If you could provide more context or clarify what you mean by "apply sin by mail", I'd be happy to try and assist you further!