Emily Dickinson Lexicon

mail, n. [Fr. maille, stitch in knitting; L. macula, spot, mesh of net.]

1. Armor; protective gear; plated wear; suit made of chains.
2. Layer; sheet; covering; [fig.] garment.
3. Phrase. “celestial Mail”: divine protection; [fig.] temple covenant; sacred endowment; heavenly investiture; (Webster's 1844 definition of CELESTIAL, adj. cites Pope: “Descending from heaven; as, a suit of celestial armor”.)