noun
later mail; next letter
Used in written correspondence to refer to a subsequent letter or mail that will follow. Often appears in phrases like 後便にて or 後便でお知らせします.
詳細は後便にてお知らせいたします。
I will inform you of the details in a later letter.
先日の件については、後便でご連絡差し上げます。
Regarding the matter from the other day, I will contact you in a subsequent mail.