How to change the default Mail account in Mountain Lion? I have 2 mail accounts configured. When composing a new message, it will default to the account for the currently selected mailbox, which is fine. However, when no particular mailbox is selected (when viewing the combined Inbox for example), it will default to the first listed mail account.

How can I change the default mail account when no particular mailbox is active? IIRC, you used to be able to drag them into order, but this no longer seems to work.

I am aware that you can specify a default mail account for all new messages, but I'd like to preserve the behavior where it will default to the account of the currently active mailbox, if one is active.
Instead of trying to drag the account(s) into the desired order in Preferences, drag the account's mailbox to the desired order from the mailbox list in the sidebar of Mail.app main window. Open the inbox and drag the desired sub-inbox to the top. The position in the other places like the trash and drafts boxes will follow.

Then quit Mail.app and open it again to make the changes take effect.

Whichever account's mailbox is at the top of the list will be the default mailbox.

If an account mailbox is at the top, but the account is not the default, drag it down and then back up to the top. This will reset the order and establish the top one as the default.