Potentially controversial opinion: business email should have exactly one person on the "To:" line.

I first heard this tip at the 2011 Inbox Love conference, I think from someone at Twilio, and it's stuck with me ever since.

If you can't send an email to only one person, it's worth asking why:

• Not sure who's responsible? Fix that, that's an organizational problem.
• Afraid they won't read it? Maybe you haven't communicated the item's importance.
• Huge status update? Use project/status-tracking tools.
• Do you regularly send to a group? Use an alias or chat system.

It's interesting how many communication pathologies are actually org problems: responsibility gaps, unclear decision processes, messed up reporting lines, etc.—fix those and the communication problems will vanish.