We do not get mails from a specific provider. Their administrator send me this error message. Is it mean the problem is on our side? I'am not friendly with Exchange, how I can fix it, if I can?
Jan 27 10:42:02 lb2 postfix_smtp/smtpd: NOQUEUE: reject: RCPT from ketamin.websupport.sk[188.8.131.52]: 550 5.1.1 <firstname.lastname@example.org>: Recipient address rejected: User unknown in relay recipient table; from=<email@example.com> to=<firstname.lastname@example.org> proto=ESMTP helo=<localhost>