This chapter was way less consistent than the ones before it.
It's an established rule that everyone gets an email every day, whether they're healthy or infected.
No one was sleeping in Saya's room with her, so even if the cult leader was able to spoof the game address, Saya should've received *two* emails that day, one saying she was healthy from the start. It should've been a point of confusion for her.
But that never came up. Why? I hope the sloppiness doesn't continue from here.
They try to explain it away at the start of the next chapter, but if this guy could actually "delete emails by hacking into people's wifi" (<-that doesn't make sense, you'd have to hack their physical device and their email server, not their fucking wifi) then he could not only build loyalty but just instantly control the entire game. He'd be faking emails to literally everyone saying that anyone he wanted to die will die and that he will never be the donor for anyone.