e-mail distribution lists are very important. When you are composing an e-mail, first set the distribution lists. They are your audience. They drive the content of your message. The "To" list is particularly important.
It is rare that an e-mail should address more than one primary addressee on the "To" list. Out of 100 e-mails, 98 should be directed to one person. Surely, there are broadcast e-mails directed to many addressees like: "Let's meet at the Cantab after work for a drink." But most of our daily messaging is dedicated to information flow with the goal of getting something done. With the goal of shipping it.
If your "To" list has more than one person, then what you hope to get done as a result of the e-mail won't. This is because all of the multiple primary addressees will more typically than not assume that the other people on the list will take next action. We are inundated with e-mails. I scan my inbox and semi-consciously filter out those for which I am not directly on the hook to reply. Save it for later, so I can deal with the 12 other messages that require immediate attention.
At the very least, a flabby "To" list will generate an unproductive flood of responses the primary effect of which is to decide who is going to take the next steps. Most of these battles take place under cover of reviewing the content of the message. But don't be fooled! The struggle is often more about who has the responsibility.
If you are genuinely puzzled about the single addressee, then you have bigger problems. You need to think again about how to refine or better target your message. In such cases, where you are still not sure, just pick someone who should care to take further steps on the issue. Put on the "CC" list all of other parties you think are interested. By doing so, at least you put the primary addressee on the spot to decide about the owner.
But don't cheat! Don't always put the boss as the primary addressee, even though the boss may have ultimate responsibility for getting the job done. By doing this, you are showing your boss that you are disconnecting. That you are not working in any way you can to find and implement a solution. Maybe, once in a while, you can send a message just to your boss, indicating that you are at a loss to say who should take action. But, if you find yourself doing this often, then you ought to have a face-to-face discussion with the boss to talk about the company's serious problem with roles and responsibilities, or what is unfortunately more likely, that the company may have a question about your role.