Apparently, not everyone thinks Dropkick Murphy Ken Casey did the right thing when he punched an audience member who made what appeared to be a Nazi salute at the band’s recent concert in New York. In case you missed it, the Dropkicks had just finished the song “Skinhead on the MBTA’’ and were about to play AC/DC’s “T.N.T.’’ when a man with a shaved head raised his arm in a “Heil Hitler’’ salute. Casey wasn’t having it, and promptly punched the guy. A video of the incident went viral. On Friday, the Boston band posted a picture on its Twitter page of an e-mail from someone threatening to assault the band during its upcoming Australian tour. (The band is playing five dates Down Under starting March 30.) The nasty e-mail includes this not-so-well-written line: “Be careful in Australia because there is plenty of neo nazis in Australia who would love to kick your [expletive]. . .’’ The band had no comment on the e-mail.