kumomta/docs/reference/events/smtp_server_mail_from.md

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

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.

{{since('2023.08.22-4d895015', indent=True)}} The conn_meta parameter represents the connection metadata and can be used to share state between the various SMTP listener event handlers. See Connection Metadata for more information.

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)