C-Mail is an idea made by Jay, or AgentCobra26. It is where you make a c-mail address on a new thread, acting similar to real emails. Commonly used on the MB, C-mail possibly stands for Cahill-mail or Clues-mail. Agents on the board with a C-mail must use their agent name or name they go by on the board as to not use their actual email. The idea became so popular, it was put in a mission.
Example: LlamaBaseball@the39clues.com is an example of a C-mail address.
C-mail addresses end in a few different ways: