It’s easy to do, and there’s no reason not to when you know that the group you’re talking to are all men. But what if you don’t know? What if you only think you know? What if the “Chris” you’ve been emailing for months is really “Christine”? What if the person you think is “Mr Robert Kirrily” is “Ms Kirrily Robert” (not that I’m bitter about that, or anything).

Better to use a more general form of address. It’s not that hard; there are plenty available, from “Hi” to “Greetings, mortals!” (Be careful, though, that there are no undead in your recipients list.)

Bonus points, by the way, to the men of london.pm who pointed out the error with a certain degree of good humour; it’s often better (and easier) for those who are not directly affected to point out rudeness than it is for those who are.