I'm pretty sure this is just another hoax. As I pointed out in the other thread about this, the only ones out there bombing mosques, throwing acid in people's faces, and
tearing off hijabs are Muslims. This letter practically reads like something some kid wrote trying to sound like it's a threat to Muslims, and I am pretty sure that's exactly what this is.
Never mind, of course, that threats against non-Muslims -- and actual acts of terror against non-Muslims and Muslims alike,
by Muslims -- far outstrip these faux threats that keep finding their way into the news.
I wouldn't worry too much about this.