Since you can play Chess by mail (See FAQ) or use a Chess Correspondence Server, I wondered how it would be possible to solve Sudoku by mail. In this case, it requires $4.50 for the 9 denominations of 1 cent through 9 cent stamps. Do not use rare stamps. You won’t be thanked. For those of you that like to solve Sudoku puzzles quickly, I believe that this variant will slow you down. The puzzle is considered neither easy nor hard.
While there are sites that claim the ability to do Sudoku by mail, this simply means that the site is willing to send you email (daily) with one or more Sudoku puzzles for you to solve on your own computer. This is convenient, but no way will a puzzle take days, weeks or months, iteratively, to complete.