I am going to address this a little differently based on the comments I read.
Let's assume that you wrote an email to your boss explaining a medical condition as a reason for absence. You certainly can put a request in the email that the boss consider the details confidential and not to pass them along. However, be aware that he might need to pass the information to HR if you are asking for some sort of accommodation or extra leave due to the condition. In that case, ask him not to share with anyone except the HR person responsible. This is how several of my colleagues who had cancer chose to handle it and most managers know that they are not supposed to pass on details of anyone's medical conditions without explicit permission from the employee.
Next let's assume you wrote something about feeling too hungover to work or still drunk from the night before. He may feel obligated to pass that on to HR whether you ask for confidentiality or not because it indicates a workplace problem they may need to deal with. Your best bet here is not to share such details in writing.
Let's assume you wrote something about how you dislike Joan because she's ugly and incompetent. Again this comes under the heading of things you don't want to put into writing. If you need to complain about Joan's performance (but not her appearance), then instead of claiming incompetence without any backup, give specific examples of work problems she has created and let the boss decide how critical they are, whether she is incompetent, and whether anything needs to be done. Assume this information will make it to Joan as she has the right to defend herself against accusations. So don't say anything that can't be proved.
Suppose you write that you are being sexually harassed. You can ask for confidentiality but only to the extent it doesn't involve keeping the information from people who need to know. For instance if your boss is not Joan's boss, he is going to have to tell her boss to resolve the issue. And he is going to have to tell HR. However, you certainly can request that the other members of your team not be informed.
Suppose you wrote an email inviting three of your coworkers to meet at the bar after work. Assume there will be no confidentiality and they will also choose to invite people maybe even people you don't like. In this case it might be best to catch the people in person and invite them and then tell them you want to only see them. You don't want to put in writing that you are excluding deliberately someone from an event. It will only backfire, hurt the person, hurt your professional reputation and possibly cause people to take sides and some will take the side of the excluded person. Anyway, this sort of thing is not work-related, so it is best not to use work channels for that sort of thing.
In general though, assume that workplace emails are not private and not the tool to use if you want to share private information.