kumomta/docs/reference/events/smtp_server_rcpt_to.md

kumo.on('smtp_server_rcpt_to', function(recipient))

Called by the ESMTP server in response to the client issuing a "RCPT TO" command. The event handler is passed the recipient address parameter from the RCPT TO command.

The recipient address is an EnvelopeAddress object.

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

kumo.on('smtp_server_rcpt_to', function(recipient)
  if recipient.domain == 'bad.domain' then
    kumo.reject(420, 'not thanks')
  end
end)