First of all they are infecting computers, routers,
dumb smart dishwasher/microwave/teapot etc and making them part of their botnet, then owner of this bot net giving command to spam exact address and here you go, they all following this command (no exact tools for that, everything really specific depending on how they are infecting them and how to give them commands). Well, this is the most easy one for the malicious purposes because for white hat one free botnets and tools for testing websites requiring
proofs that you actually allowed to do so.
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