While you can easily rearrange the order of your accounts in Mavericks’ mail sidebar, when composing a message there is no easy way of rearranging the accounts as shown in the “From:” dropdown box in a new message; accounts are listed in the order in which they were added, which, in the case of using iCloud Keychain, may be vastly different than the order that you want them in.

Fortunately, there is an easy procedure to rearrange them:

Open up the “Internet Accounts” system preference pane
For each email account, in the order you want them to appear, uncheck and then re-check “Mail”

In this way, the accounts will be removed and readded to Mail in the order that you want them to appear.

Source: Mac OSX Hints