What is the best explanation for why a FortiMail unit would issue the error message indicated in the exhibit?

Question

Exhibit

Options

• AThe recipient domain external1.lab is not defined.
• BThis traffic comes from an authenticated sender.
• CRecipient verification is not working properly.
• DThe session is matching an Access Control Rule with action "Reject".

• A
  81% (57)
• B
  3% (2)
• C
  4% (3)
• D
  11% (8)

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