I learned an interesting French word last night: maille [maj], which means stitch or mesh and appears in such expressions as:

– maille à l’endroit = plain stitch
– maille à l’envers / tombée / coulée = purl stitch
– maille Jersey = stocking stitch
– doublure maille = mesh lining
– maille du tricot = knitting stitch
– maille du crochet = crochet stitch
– à maille serrée = close-woven
– avoir maille à partir = to be in trouble
– avoir maille à partir avec qn = to have a brush with sb
– à mailles fines = with a fine mesh
– passer à travers les mailles du filet = to slip through the net
– cotte de maille(s) = coat of mail; chainmail

Maille comes from the Old French maille (loop, stitch, mesh, link), from Vulgar Latin *macla, from Latin macula (spot, speck, stain; mesh; cell) from From Proto-Italic *smatlo-, from Proto-Indo-European *smh₂tlo- (possibly meaning “wiping”).

The English word mail, as in chainmail, comes from the same root via the Middle English maille ‎(mail armour) the Old French maille.

The English word mail, as in letters and parcels, originally meant a bag or wallet, and came to mean a bag containing letters to be delivered by post, and then the letters themselves. It comes from the Middle English male, from the Anglo-Norman male, Old French male ‎(bag, wallet), from the Frankish *malha ‎(bag), from the Proto-Germanic *malhō ‎(bag, pouch), from the Proto-Indo-European *molko- ‎(leather pouch).

*molko- is also the root of the French words malle (large suitcase, trunk) and mallette (briefcase); and the Spanish mala ‎(suitcase, mailbag, mail, post), and maleta (suitcase).

Mail (letters) in French is (le) courrier and the postal service is la poste. Email is officially courriel or courrier électronique, though many people use e-mail. Courrier is borrowed from the Italian corriere (messenger, courier), from correre (to run, hurry, rush), from the Latin currere, from currō (to run, hurry), from Proto-Italic *korzō (to run), from Proto-Indo-European *ḱers- ‎(to run), also the root of the English words courier and current.

Sources: Reverso, Wiktionary