If you have multiple email accounts, you can set which account a message is sent from. By default, messages are sent from the account of the last viewed mailbox.
Using Mail Preferences, you can choose a specific account from which all email messages are sent. You can also display a pop-up menu in your new message windows that lets you choose a different account for each message you send.
To set the account from which to send all messages, choose Mail > Preferences. Click Composing and choose the account from the "Send new mail from" pop-up menu.
To display the Account pop-up menu in new message windows, choose File > New Message. Click the Action pop-up menu in the lower-left corner of the header area and choose Customize. Select the checkbox next to Account, then click OK.
To change the order in which accounts are listed in the mailbox list, drag the account mailboxes into the order you want in your Inbox. You can also rearrange the account order in the Accounts pane of Mail Preferences.