Change the default Macmail account for OS X 

Depending on the version of OSX, the following address this question:

Mac OS X 10.0 through 10.1.5

The default account when composing a new message in Mail is the one listed first in the Description column of Mail Preferences. To change this default, you only need to change the order of the names.

Follow these steps:

1. Choose Mail > Preferences.
2. Click Accounts.
3. Drag account names in the Description column to change their order. The account that is listed first is the default account that is used whenever you open a new mail window.

Mac OS X 10.2

The "From" address for a new message will generally be the same as the "To" address of the message that is currently selected. If no message is selected, then the "From" address associated with the currently selected mailbox will be used, if there is one. Otherwise, the default "From" address will be used as described above.

Note: You can also change the account used to send a message on an individual basis by using the "From drop down menu" in any new message as it is being created.