unsealed mail; open letter
Refers to mail or a letter that is sent without being sealed, often for convenience or to indicate non-confidential content.
開き封で書類を送ることもあります。
Sometimes we send documents as unsealed mail.
Compound of 開き (hiraki, 'opening') + 封 (fū, 'seal'). The exact historical derivation is uncertain, but the term literally describes mail that is left open.