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
recipients. This is a permanent error. The following address(es) failed:

  kajoslav074 na tiscali.cz
    SMTP error from remote mail server after end of data:
    host tcx.spamfree.cz [94.242.232.68]: 554 message refused (#5.7.1) - for more information visit https://www.virusfree.cz/cs/help
  iloveyumyfriends21 na gmail.com
    SMTP error from remote mail server after RCPT TO:<iloveyumyfriends21 na gmail.com>:
    host gmail-smtp-in.l.google.com [74.125.133.26]:
    550-5.2.1 The user you are trying to contact is receiving mail at a rate that
    550-5.2.1 prevents additional messages from being delivered. For more
    550-5.2.1 information, please visit
    550 5.2.1  https://support.google.com/mail/?p=ReceivingRatePerm b1si2752018wrh.515 - gsmtp