The "Send" server that Postbox will use is driven by the following conditions:
- If a single account is selected in the Accounts pane, the compose window will use the send server preference for that account. This can be set in Preferences|Options > Accounts > [your account] > Identity.
- If an Account Group is selected in the Accounts pane, the default send server for the account listed at the top of the group will be used.
- The "default" send server will be used in the following circumstances:
- When the main window is closed or minimized and the keyboard shortcut for New Message is used
- When the "Local Folders" or "On My Mac" account is selected
- When clicking a mailto: link in a browser
The default send server is determined by the topmost account listed account in the Accounts Pane. If your Accounts Pane contains one or more Account Groups, the topmost account in the topmost Account Group will be considered the default.
To change your default account in Postbox, from the main mail window drag-and-drop the account you wish to set as the default to the top of your account tree, or to the top of the topmost Account Group.