Mail delivery failed: returning message to sender

[Mail Delivery System]

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A message that you sent could not be delivered to one or more of its
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  kocvara na kovosvit.cz
    SMTP error from remote mail server after end of data:
    host bx.virusfree.cz [5.180.49.34]: 554 message refused (#5.7.1) - for more information visit https://www.virusfree.cz/cs/help