kumomta/docs/reference/events/smtp_server_mail_from.md

kumo.on('smtp_server_mail_from', function(sender))

Called by the ESMTP server in response to the client issuing a "MAIL FROM" command. The event handler is passed the sender address parameter from the MAIL FROM command.

The sender address is an EnvelopeAddress object.

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

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