You can drag the order of accounts in Mail preferences in the Accounts pane. Put your preferred account as the first.
Also, in the outgoing server preferences, check the box to use only the server for the selected account. Make sure the correct server is showing.
Also, if you highlight the account you wish to send from in the sidebar, that should be the active account when creating new messages.
In addition: Check the mail preferences under composing and make sure you don't have an account selected to send new messages from, or select the account you wish to always send from.