[Linuxargentina-general] Undelivered Mail Returned to Sender

This is the Postfix program at host app.realspektrum.cz. I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. For further assistance, please send mail to <postmaster> If you do so, please include this problem report. You can delete your own text from the attached returned message. The Postfix program <[email protected]>: host 192.168.171.65[192.168.171.65] said: 550 5.7.1 Relaying to <[email protected]> denied (authentication required) (in reply to RCPT TO command)