When you create a new email message in OS X Maverick’s Mail app, you can choose–assuming you have more than one email account in there–which account you’re sending the email from. For example, you might want to send an email from your work account rather than your personal one if it’s work related, and vice versa if it’s about a party you’ve recently attended.
The problem is, when you choose from the drop-down menu in the mail composition window, the account you want to send from may not be in the top spot. It might be a couple of slots down the list. If you want to rearrange the order of these accounts, you can search in the Mail preferences until the cows come home because the ability to do so just isn’t in there.
It is, however, possible to do..