Google / SMTP error code 550
smtp;550 5.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. For more information, please visit https://support.google.com/mail/?p=ReceivingRatePerm - gsmtp
High severity
While this could indicate a busy or inactive Gmail user, sometimes this indicates the recipient is being 'mail bombed' - a tactic used to disable a victim's mailbox in order to hide malicious activity. Please verify the recipient was not added to your list by an automated script or other malicious entity.
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