The rule: Directly address each person included in the thread so they know exactly who is expected to respond and/or act.
Writing a request or question to multiple people without clearly calling on a specific individual leads to "diffusion of responsibility.” Psychologically, a person is less likely to take responsibility for a request when others are present because the individual assumes that someone else is going to take care of it (or has already done so).
Here’s an example of an email that could easily lead to diffusion of responsibility:
Please send me the summary of today's technical meeting about the battery design. I also need someone join me in the lab tomorrow morning to test the new battery prototype.
And here’s how to avoid it and make sure things get done:
John, please send me the summary of today's technical meeting about the battery design.
Robert, please join me in the lab tomorrow morning to test the new battery prototype or make sure someone else from your team comes if you're not available.
Key takeaway: People aren’t going to volunteer for extra tasks, especially if they can assume someone else will take care of it. Bypass “but I thought Tina handled it” by specifically asking George to tackle the issue.