Official Gmail Blog: “M” is for…mute:
Just like you, I get a lot of email. Much of it requires my reply, some is simply FYI, and some is the result of overactive mailing lists, like the one for Google’s San Francisco commuters. I take the shuttle to work, so I need to stay on top of announcements about route and schedule changes. But when there’s an alert about a route on the other side of the city, or a co-worker’s comment spurs a long thread about bus etiquette, I use the “m” shortcut key to mute the conversation and spare my inbox. As new messages are added to the same conversation, they bypass my inbox. If someone puts my email address directly on the To: or cc: line, the conversation immediately re-appears; otherwise, irrelevant messages sent to the mailing list are archived. That way I can refer back to them when I really need to find out about changes to the route when there’s a Monday night game at the 49ers stadium, or want to revisit the debate about dogs on the bus. To use the “m” shortcut key, first click “Settings” in the top right corner of Gmail and select the option “Keyboard shortcuts on.” Then, when you’re reading a conversation you want to mute, just press “m.”
I had no idea. And is that ever useful.