Sergeant's Mail is a dropped item set, found only in Wayrest Sewers. It cannot be crafted. Set pieces are Redguard style in Heavy Armor.
2 items: Adds 14-1206 Maximum Health
3 items: Adds 1-129 Health Recovery
4 items: Adds 1-129 Weapon Damage
5 items: Increases the damage of your fully-charged Heavy Attacks by 19-1634.
The set consists of heavy armor in all slots.