kumomta/docs/reference/events/smtp_server_ehlo.md

kumo.on('smtp_server_ehlo', function(domain))

Called by the ESMTP server in response to the client issuing either a "HELO" or "EHLO" command. The event handler is passed the domain parameter from the HELO/EHLO command.

You may choose to reject the connection via kumo.reject.

-- Called to validate the helo and/or ehlo domain
kumo.on('smtp_server_ehlo', function(domain)
  -- Use kumo.reject to return an error to the EHLO command
  if domain == 'bad.actor' then
    kumo.reject(420, 'go away')
  end
end)