Sergeant's Mail is a heavy armor set. The 5-set bonus increases the damage of your fully-charged Heavy Attacks by 1634.
The Sergeant's Mail set is part of the base game. It drops in Wayrest Sewers I and Wayrest Sewers II in Wayrest. Each set item is bound on pickup. You can acquire heavy armor, weapons and jewelry of this set in the Aldmeri Dominion, Daggerfall Covenant and Ebonheart Pact motif style.