It’s not the first time a letter has gone viral, splitting Twitter down a bitter divide, with resentments amassing on each side. And this letter didn’t even concern anything so contentious as Brexit, Marmite, or whether we need the royal family. Instead, it was part of the Guardian’s “A letter to” series – taglined “the letter you always wanted to write” – in which anonymous authors vent cathartically about topics they can’t, for whatever reason, broach in person.