Click any content in the mail sent by other player and you will find the “Block” button. Click it to block mails from this player and that player will be in your blocked users list (click your Avatar→ Settings → Blocked Users).
If you want to restore communication with this player, unblock in the Blocked Users by click the “Unblock” button.
Please note: The “Block” function blocks communication only, it will not prevent you from being attacked by other players.
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Game Guide for Clash of Kings