Helo command rejected: Access denied (in reply to RCPT TO command)

Dobrý den,

zákazník nemůže odeslat email, vrací se mu to s tímto textem – Netuší někdo, kde je problém? od ostatních zákazníků je to v pořádku, předpokládám, že je problém u tohoto konkrétního, nicméně se jedná o docela důležitého zákazníka, proto bych rád věděl, jak to nastavit/opravit:

This is the mail system at host <a href="http://dd20302.kasserver.com" target="_blank" rel="noopener" data-saferedirecturl="https://www.google.com/url?q=http://dd20302.kasserver.com&source=gmail&ust=1601468624628000&usg=AFQjCNHYl9iIu5DiprtQAiQfHBE8ss-OYQ">dd20302.kasserver.com</a>.

I'm sorry to have to inform you that your message could not
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For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
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                   The mail system

<mail@domenauWedos.cz>: host <a href="http://wes1-mx1.wedos.net" target="_blank" rel="noopener" data-saferedirecturl="https://www.google.com/url?q=http://wes1-mx1.wedos.net&source=gmail&ust=1601468624628000&usg=AFQjCNHdRTk_lw40gIVwcGovlIhn79tsOA">wes1-mx1.wedos.net</a>[46.28.106.<wbr />11] said: 554 5.7.1
    <<a href="http://dd20302.kasserver.com" target="_blank" rel="noopener" data-saferedirecturl="https://www.google.com/url?q=http://dd20302.kasserver.com&source=gmail&ust=1601468624628000&usg=AFQjCNHYl9iIu5DiprtQAiQfHBE8ss-OYQ">dd20302.kasserver.com</a>>: Helo command rejected: Access denied (in reply to
    RCPT TO command)