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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  petr na kpt.cz
    SMTP error from remote mail server after end of data:
    host antispam.smarthost.cz [130.193.9.47]: 550-This message <6B6ED88B80547AA97570398581DB685B na linux.cz> was classified as
    550 SPAM.