SELECT
EmailAddress, AgentName, AgentNumber,
  CASE
    WHEN AgentNumber = '6006-LK' THEN 'LK@email.com'
    WHEN AgentNumber = '6006-MM' THEN 'MM@email.com'
    WHEN AgentNumber = '6006-MT' THEN 'MT@email.com'
  END as SendingEmail
FROM TestDataExt