A formal, honorific closing formula used at the end of letters, equivalent to 'Yours respectfully' or 'Sincerely yours'. It is a yojijukugo and belongs to sonkeigo register.
Write 'Tonshu saihai' at the end of the letter.
A yojijukugo composed of 頓首 (bowing deeply) and 再拝 (bowing again), expressing profound respect. The exact historical derivation is uncertain, but it has been used as a formal letter-closing phrase.